you have to enter this room, there are two ways to go
1. find the lockpick and open the door lockpick is in the class on the left, on the second bench on the left, down then use it in the door and a mini-game with opening the door will start
2. In the class on the right, neat the benches, find the book and give it to the girls (you can also drop the purse to get the key and use the key
there is Curtains take it next to the boxes or take it from the blackboard and use it in the middle class I think it will help you