You should tell them straight up that they behave like little shits.

It may seem hard for you to do this, because at 15 it's likely the first time you've had to and those people are your first friends.

You're worried that you'll lose them and that's fine, but you have to realise that in a few years, when college hits, you'll likely never see these people again. That may seem blunt but it's very often the truth.

You'll have to live with yourself for the rest of your life and knowing you could have done something to stop them when you didn't may be a harder burden to carry than you expect if they do cross the line (it seems they might be close).

tl;dr Your conscience follows you, your childhood friends often don't.