When I wrote two high school students, I didn’t put in cursing in, but I personally didn’t want to see it in my writing, but then realized that wasn’t how my kids talked, except around me and other adults (mostly). Around their friends I’m sure they swear up a storm.
Although that’s probably true (heh), you don’t need to literally include cursing to make teenagers sound real. Network TV doesn’t have a lot of cursing, but it has widely accepted representations of teenagers on a bunch of shows. Watch some TV for inspiration! Bad grammar, apathy, and crass-but-not-cursing language can all sound realistic.
“Ew, get away from me.”
“Wha? That sucks!”
“I’m not gonna eat that crap.”
“Can you believe she said that? She’s THE WORST!”
Even after making some adjustments, it took me a while to get used to it. I guess I just find it hard to be casual with my dialog sometimes. I’ve done so much formal writing that it’s hard to break.
Another potential exercise: write as comfortably/naturally as possible for a few pages, and at the end, literally copy and paste ONLY your dialogue lines into a separate document. Then write 2-3 alternatives for each dialogue line before deciding what to paste back in.