I believe the guy and girl accidentally meet up in the school's pool by breaking in. After they hook up, they both fall asleep in the sun. When they get to class, their classmates notice they have been tanned and sunburned. Everyone finds out that they hooked up because the girl has a cock and jizz tanned on her leg. If anyone can remember the name of this manga, i really appreciate it.

Got help elsewhere to find title, Slippery School Play.