I can't seem to remember the manga(hentai). I'll try to describe it as best I can. The story is set in a time where students in school attend this sort of class about sex. The teacher announces that you are paired up with a girl/boy. The boys are ecstatic about the development while the girls seemed more worried and the most popular girl(wears a pin i think, has short hair) reprimands them. The main character gets paired up with the most popular girl in their class, but laments about how her bad attitude(shes a tsundere). They line up to go into rooms provided within the school and their job is to collect semen(with the condoms provided). So the popular girl(begrudgingly) gives the guy a blowjob and he blows his load into the condom. But the guy inturn returns the favor. The setting changes to the guy leaving school. And the girl catches him and drags him into the bathroom where they continue from where they left off. They have sex in the toilet. The girl agrees to become his "partner" and that they would have a better chance of improving their grades/score since they are compatible. It ends there I think.

I don't remember the name of the artist in the slightest.