There are 4 requirements you must do in order to achieve the True Ending. You must start to do them when you get the staff room card and you must also have them in order to enter the room to talk to the Noteman for the final one. Here is how to do each requirement:
Raise Your Social Studies Grade: When you get to the staff room, you will see a shimmering spot on a message board to the left of middle of the room. Investigate there and you will find a sheet of paper with the codes for all the classrooms. Return to the West Building and head to 1C on the first floor. Once inside, head to the middle of the room to find your friend Mina. She is bleeding and will ask you to go to the Medical Centre to get something to help her. Return to the Main Building, then head to the Medical Centre and around the middle of the room you will find a medical kit. Return the Mina and use the kit on her to heal her. Later on when you make it to the North Building, you will encounter Myung-gil. Approach him and start the conversation with him and eventually Mina will show up to help you out. When the conversation ends, your Social Studies grade will go up.
Raise Your Math Grade (without cheating): In the Main Building,(progress the game until you unlock the north building and talk to Myung-gil) then head to the 4th floor and enter room Study A. Go to the middle of the room and investigate the chalkboard there to see some math problems. Leave the room and had to 3F next door and talk to the student chained to the desk in the back. He says he will help you with math problems if you get him the key to Mr. Ma’s office. Head back to the West Building and go up to the 3rd floor into room 2F. Look on the teachers desk to the far right of the room to find the key. Return to the student, give him the key and he will show you how to do the math problems. Return to the Study A room next door and investigate the chalkboard again. This time you will figure out the answer and your math grade will go up.
Raise Your Art Grade: In the North Building, head up to the Art Room and look at the file holders on the table in the middle of the room. You will note that you never turned in your art work and that you hid it in your homeroom. Make your way back to the West Building and go to your classroom of 1E. Go to the very back wall of the room and get the pickup item there to retrieve your art project. Return to the Art Room and submit your art work into the file holders again to raise your art grade.
And Finally :
Talk To The Noteman During Your Escape: When you make your way through the Main Building while trying to escape (final sequence) , head to the door to the cafeteria, but don’t go through it.
Instead, go up the stairs there to the 4th floor. Once there, go to the right and through the double doors there. Walk over to the right and talk to the Noteman. Use the following responses/questions when you are talking to finish off his requirement:
Show him the pendant
Ask him about the girl in the picture
Ask him about the shade
Ask him how to escape
Once you have achieved all of these objectives, you just need to make it to the West Building and get back to your classroom of 1E and talk to Yaesol there. After you speak to her, the True ending will occur.
By Villain