This is the first achievement you get once you start playing. Just press C or Shift to take a Quicksave (it doesn't count as a permanent save data)

Check your inventory for the first time, both Items and Key Items are carried here (you'll find them as you progress your game). Click X to open their menu (using X to ineract with someone or something will let you use your items on them, eventually)

Reach the Hall on the 1° floor. There are two bathrooms on the right side (interacting with the blood stain on top of the wall will trigger a game over). Enter one of the bathrooms and check a toilet, male or female doesn't matter (if you enter the girls' one, don't knock on the door of the locked stall).

You have to summon little Hanako to trigger this achievement (meeting up with her is story related anyway, therefore it can't be missed). Take a Quicksave first and enter the girls bathroom on the 1° floor. Knock three times on the door of the locked stall at the end of the room, and wait for a text line. Take a few steps back and let Hanako exit the stall, she'll try to give you a hug, avoid her and exit the bathroom (let her reach you to trigger a game over). Re-enter the bathroom and (still avoiding her) examine the toilet of the stall she came out of, and get both an item and the achievement.

Have a meal. Easy right? Reach again the Hall of the 1° floor, go down to see the Infirmary (right) and the Cafeteria (left), enter the Cafeteria. Take a Quicksave. On the left side of the room there's a table with an interactive meal, and you'll be prompted if you want to eat the food. Answer yes and enjoy the scene.

This one scared the life out of me. Go to the left on the 1° floor over the Faculty Room and you'll enter another Hall. Right up from you a corpse falls down from the ceiling. Take a Quicksave, approach him and let him grab you; he will force you to jump off the rooftop but you'll get the achievement.

Reach the Library on the 2° floor and talk to Miss Library for the first time. She will ask you to be her friend, accept her invitation and you'll get the chance to talk to her again (refuse persistently her request to get a game over). Tell her your name, too. Now you can Talk to her whenever you want, or ask for Hints. Go for the Talk option at least three or four times until she starts to repeat herself, that will trigger the achievement.

Read the Secret Diary of Aya, from Mad Father by interacting with the bookshelf (Library, 2° floor), and if you're lucky the achievement will be triggered.
Update: the achievement seems to not be bugged anymore.

Enter the Library (2° floor) and go to the right upper corner to enter Miss Library's room. Interact with the bookshelf and read "The Lazy-Eyed Ranger" to get the achievement (picking "don't read" will trigger a game over)

Reach Yoshino in the Science Room, where she's tied up on a table. You get there by going through the Laboratory on the 2° floor, but something like a laser is placed between you and the Lab (stepping on it will trigger a game over). Follow this steps:

• Pick up the CD-ROM on the floor, right in front of the laser (Lab)
  
• Go back to the Computer Room (3° floor)
  
• Disable the security system by interacting with the working pc, use your CD-ROM (increasing the security system will trigger a game over) to obtain the access.
  
• Laser is off now. Get back to the Laboratory and take a Quicksave. On the lower corner there's the entrance to the Science Room, interact with Yoshino and she'll ask for help, refuse and leave her to her fate.

Now you have to help Yoshino in the Science Room. If you still have to get there follow the instruction for the "Savage" achievement (and take a Quicksave). Instead of rejecting her request of help, agree to free her. The kidnapper is about to come back to the room, you have to hide somewhere in order to avoid getting caught. Close to the Bear there are a bunch of cardboard boxes, interact with them and hide inside one (you will trigger a game over with your achievement)

You have to help Kudoh, a classmate of yours trapped in the Gymnasium with a broken leg. To open the door you need to obtain the Emergency Key, stuck inside a vase in the Principal's Office (4° floor). You can't just interact with the vase and take it, or you'll trigger a game over because of the poisoned roses. Follow this steps:

• Get in the 1-A classroom on the 1° floor, going left from the main Hall and passing over the corpse on the ground.
  
• You're in class now. Avoid the shadow on the floor and reach the upper side to get a Metal Bat.
  
• Come back to the Principal's Office (4° floor) and use the Metal Bat to smash the vase and get the Emercency Key (use X to interact with it).
  
• Reach again the 1° floor, this time go to the right (over the bathrooms), take a Quicksave and enter the tiny black door in the right corner.
  
• A Spirit is now following you, and you have to go straight up without looking at him in order to avoid a game over (don't approach the other Spirit you'll see).
  
• Unlock the door with your Emergency Key, go up and meet with Kudoh.
  
• Start a conversation with him and when asked if you wanna watch or help, try to help him to get the achievement (and a painful death)

To access the Music Room and the Piano you need to get yourself a bunch of Salt, which can be crafted from the Salt Mandrake by bringing it to the Electric Grater. Follow this steps:

• Reach the Science Room (see the "Savage" achievement to get there) and talk to Yoshino, and she'll beg for your help. Make Yoshino promise to never bully someone again. She agrees, but the Doctor is coming back.
  
• At the top of the room there's an interactive locker, try to hide inside it just to find out you don't fit; give up when prompted. Take the Scalpel from the opened locker, approach the Bear lying on the third table and use it to rip it open and hide inside the costume (or better, the Bear's corpse)
  
• Upon the Doctor entering the room, listen to him murdering Yoshino. Jump out the Bear and interact with Yoshino's soul to trigger the final scene.
  
• After Yoshino's murder scene, take a Quicksave and exit the room, then check the working machine in front of you (was off before the episode)
  
• Free the Salt Mandrake by interacting with the machine.
  
• Read the Book about Mandrakes (Salt and Bomb) without going anywhere near the telephone on the table (otherwise you'll trigger a game over) while the Mandrake wanders around the room.
  
• After reading the book, walk around in order to push the Salt Mandrake close to the telephone (be careful), this will make her faint and let you pick her up.
  
• Reach the 1° floor, Cafeteria.
  
• There's a zombie (?) here chopping off some kind of meat on the working table, walk over him and interact with the Electric Grater nearby (thank you Tempestus0079 for the tip). Obtain Salt.
  
• You can now finally get past the Loop Hall. Get there from the 2° floor and take a Quicksave.
  
• Be careful, don't walk near the windows or you'll trigger a game over. Trying to walk in the hall will take you back no matter how many times you try. When a Spirit comes off one of the windows behind you, interact with said window and put your Salt here, the Spirit will now let you through.
  
• Take a Quicksave, enter the Music Room. At last, play the Piano right in front of you.

You need to read a certain message left in the Secret Room, in the Laboratory. To get there you have to go past the Spirit hidden at the top of the Laboratory, which attacks you whenever you try to interact with it. The Spirit can be scared off, but you need a Small Key and a Chest to do so. Follow this steps:

For the Small Key:
• Who provides you with the Small Key is Tohma, you can find him in the boys bathroom (1° floor), locked in a stall, and trembling. Knock on the door and talk to him, you'll leave the bathroom together. Or not. He is suddenly attacked by something, getting injured.
  
• Bring him to Miss Library (2° floor) and she will take care of Tohma by letting him sleep in her bedroom (on the right side of the Library)
  
• Leaving Tohma behind, get back on the 1° floor and enter the Cafeteria. Use the Coins you got from the Hanako encounter (see "Repulsive" achievement) on the vending machine to buy some tea. You'll get an Energy Drink instead.
  
• Go back to Tohma and offer him the drink. He accepts, but something it's in the bottle: he spills out the Small Key.

For the Chest:
• Reach the Music Room, 2° floor. If you don't know how because of the Loop Hall, refer to the "Play the Piano" achievement.
  
• Quicksave and take a seat. The Piano will start playing by itself, listen the song without moving or you'll trigger a game over (by the hand of the Spirit who's playing the Piano)
  
• The Blonde one will give you something after her performance and thanks you for your time. You're be rewarded of the Chest.

You got everything you need now. Use the Small Key to open the Chest and obtain the Bell. Take a Quicksave. Go back to the Lab (2° floor) and approach the Spirit at the top of the room; the Bell will ring on her face, making the Spirit run away. Follow her in what it seems to be a Doll Room, and take a Quicksave. Interact with everything in the room (if the doll lying down opens her eyes, don't step close to her to avoid a game over). Enter the last room, where the body of a young guy (or girl) is floating into a capsule (probably your female / male counterpart as the achievement says, the one you didn't pick at the beginning of the game), read the message on his back.

This is one of the 3 endings avaiable (true, normal and bad). To get the Bad Ending just put your Six Key Items in the shrine (Courtyard, 1° floor) and as a last sacrifice (three graves out of four should be sparkling) choose Tohma. Get back to the shrine. Someone will follow you and a scene will be triggered.

Get the Normal Ending with both protagonists, Male and Female. Collect the Six Key Items and put them in the shrine. Now you have to choose a sacrifice to lift the curse, pick Mr. Kurata. and go back to the shrine. Getting the Normal Ending will trigger the Truth (the Male event is a little longer than the Female one, but it's irrelevant)

While in the Truth, interact with Yoshino's grave and trigger her event. After a couple of interaction she will be surrounded by three copies of Misao, beating Yoshino to death in order to punish her for her behaviour and get revenge. To stop Misao, turn around and pick up the three items hidden in the three toilets (Salt, Metal Bat and Coins). Give the Coins to one of Misao's copies, and trigger the achievement.

While in the Truth, go to the right side of the Dark Space and interact with Mr. Kurata's grave. You'll see a bunch of different events and also will play as him for a few scenes. You're now young Kurata, and everything about his dark past is revealed. Watch until the end to get the achievement.

Complete the Truth with both protagonists, Male and Female.
You can watch all five events of the Truth with Tohma's one as your last.
There aren't any specifical requirements or choises to take on this route, just follow the events and watch everything until the end.

• If you triggered the Yoshino episode before freeing Mr. Kurata from the locker (2-B classroom), you have to clean yourself from the blood first, or he'll think you're a monster and kill you. On the 1° floor go north and enter the Courtyard, there's a small lake with a shrine at the top. Use the water to wash yourself from the blood. (this will also open the shrine's door)
  
  
• On the 1° floor, where the exit is, there's a pink piece of paper on the wall; all the different deaths you're experiencing through the game are listed here (missed ones are marked with ???).
  
  
• When you aren't sure about what to do, ask Miss Library for Hints. She will tell you something useful to keep progressing the story.
  
  
• If you encounter bugs on the second protagonist you're using, like watching events you aren't supposed to watch (and then spawn somewhere else), open the game folder and delete ALL your save datas. Restart the game from zero directly with the character you didn't play with yet. The route won't give you issues anymore.
  
  
• If you wanna speedrun the game or just skip something you already watched, keep pressed Esc or Canc during dialogues.