As you can see from the Marauder's Map (seen in the end credits of "Prisoner of Azkaban") certain elements of the student body are clearly up to naughtiness.
That being said, although the creator of the end-credits is adamant that they're not having sex.
"Maybe it was meant to be Harry, but we've all been kids, we've all been in school and stuff ... It was just a sort of little peck on the cheek," assured Wetherell.
In Wetherell's mind, the couple's feet "are in an embrace" and "not having sex as everyone says."
Out of universe, JKR spoke about the apparent lack of ('ahem') physical intimacy in her books;
"The thing about fantasy—there are certain things you just don’t do in fantasy. You don’t have sex near unicorns. It’s an ironclad rule. It’s tacky." New Yorker Interview 2012
