Go for it. I just finished reading the YA novel about fifteen year olds, so they're in high school level. In all honesty, it doesn't make sense for high school kids NOT to swear. Even the "good kids" swear nowadays, with of the proper exceptions, of course. You hear it all the time from your friends, your classmates - heck, even some teachers. And kids appreciate that - the teachers who realize their students are mature enough so they don't have to censor themselves. I think being the author of the novel they're reading works that way, too. If it "feels" like your characters should swear, please don't hold back. (I wouldn't use it often, though, only when necessary, and even then the worst I'd use was "sh*t.") There's nothing more unrealistic to a kid's ear than to hear, "Gosh darn it, Maggie, we missed the bus!"