A while ago i read somethung about a guy who sleeps with a girl just for sex while cheating on hes girlfriend. The girl is ok with this and enjoys sleeping with the guy at the school. During thier escapade the guys girlfriend calls and they answer. It ends with the girlfriend walking into the "storage area" while they are doing it. If it helps the girl looks like a typical kuudere girl.
The art style looks like bobobo but its not him.