I really like the game. However, as good as it is, I want to throw some ideas into the table that hopefully you will found useful.
First is about the art. I really think the girls are cute and hot, but the truth is they lack characteristics. I would recommend give them more personality playing with the way they dress and the facial expressions. For example: short hair, glasses, a book in hand and a serious face for the studious type of girl; long black hair, pantyhose, a beauty mark on the face and a mischievous face for the sadistic girl; long sleeves, hair that hides one eye and a worried face for the shy girl.
Second is about the gameplay. The game right now can be easily finished with no challenges or objectives, this is nothing bad if you want to see all the girls in the lockers but I think the lack of these also takes away the gratification of playing after a while. I recommend you use a score, for every girl you put into the locker you get points, when you reach certain scores you unlock new girls in the hallway. Also, give the game some enemies to be vigilant of, they can be a teachers or more tough girls that try to break free the girls you capture and then they destroy the locker and decrease the score, giving you something to be aware of and punishing you if you don’t play by the rules. You can also put surveillance cameras in the hallways that alert the enemies or even lights in the hallway that goes off making it hard to see.
Well, that is all I can think of. I hope this comment was useful. I’m really looking forward to see the next update.