I think you're just going to have to do your best to ignore it. There's this common yet stupid belief that if a female gets kicked between the legs there's nothing there to hurt. This is obviously moronic because there's plenty there to hurt. You can't teach much of anything to the average teenager because their brains (no offense intended) are literally not completely formed anyways. And even if you could intellectually manage to teach them some common sense, they don't want it - because the point is that they want to bully you. You're an easy target because you're nice. And they'll make up whatever shit they can to keep it going.

I know this from personal experience. I had to stand in front of a class to deliver a speech and because I didn't wear my jeans so tight I had camel toe like the other girls often did, there was some extra cloth near the crotch that kids claimed was clearly due to my having a penis. Never mind the fact that I changed in front of them in gym class and didn't have a dick, and had tits. It was just fun for them to pretend I was actually a guy to try to make me cry. I didn't, I ignored it, they moved on. It's not as fun to make fun of someone who just doesn't give a fuck because you're being so goddamn dumb they can't be bothered to take you seriously. They alternated between calling me "dyke" and pretending I was a guy. I think the funnier thing was just how obsessed with my crotch they were. If you want to say anything, ask them why they're so interested in your vagina because it's a little weird.