S na Kanojo

After school, Matsumura Minoru told Shimura Saki how he feels, but left there without hearing her answer because he could not bear to be there any longer.
Next day, as walking down the hall, he heard a strange voice from a stock room. He peeped through the window and saw Saki undressed, masturbating alone.....
Minoru thought she was not aware of him then, but she called him out after school.....

[From DLsite English]