So you punished everyone for something two 12-year-olds may or may not have done? They could a) be lying and b) even if they're not, that's a cue to talk to their parents about your rightful concerns. 12-year-olds who brag about having sex just do so because they've been taught by all of society and media that men sexually desiring them is their permission to exist and the ultimate goal to follow and in some cases the only kind of affection they receive. That's sad enough, but at least you realistically taught the girls that ALL of them will get punished because you disagree with SOME of them and how they think they need to express sexuality. Instead of taking the opportunity to talk to your kid and what she thinks about their behavior and her own self-worth.

as long as it wasn't your daughter, I would have left it go, then quietly mentioned it to the girls parents later