Of the few times I’ve done this, only one of them ended somewhat well. I saw a guy deadlifting with a mixed grip and pulling up with his arms at lockout. This guy was probably in his 30’s so a modicum of maturity was present at least. We were in the gym around the same time enough for him to have talked to me about my deadlift in the past, so there was at least some level of respect toward me.
When I saw him doing that, I simply stated in a matter of fact tone “You’re free to not listen, but every time you do that you’re risking tearing your biceps.” He asked me to clarify what I meant and I explained it a little clearer. He was super thankful and managed to stop. That’s 1 of maybe 3 times that I’ve done that. Had he been a high school kid that I had never interacted with before and didn’t have any level of credibility with, I’m pretty sure it wouldn’t have gone so well.
Pro-tip: Go straight AAF on them. They’re little more than chimps with drivers licenses at this stage. Establish your dominance by pressing whatever they’re squatting with overhead. Then give any advice in the following format: ABBAB (Always Be Berating and Belittling). When you see them failing to do the exercise correctly, inform them of how “bitches” always have a hard time doing that exercise correctly and then demonstrate how it is done. This may be your only option with high-school boys.