Most of the advice here is written as if OP and his friends are 20 years old. Please read the post before you toss out advice, OP has already said he told them to stop it, that it was wrong, and they didn't listen. So suggesting "just talk to them about it" is nonsense advice. They aren't going to listen. When the majority of them agree on something, that's it, that will be the groups status quo. These are 15 year old kids... not adults, stop assuming they can deal with being called out in a sensible and mature way.

OP I'm not sure talking to her Mom will help much, but you could certainly reach out to her. Let her know you're embarrassed by how your friends acted, that it is wrong, and that you'll try your best to stop it in the future. All you can do is offer her assurances that not everyone is a piece of shit, and try to change your friends behaviour. Don't sit them down for some "heart to heart", you and mean both know it won't do anything but encourage them to ostracize you. You can do more good by gently steering them away from their terrible behaviour. I know this isn't the "ONE CONVO AND DONE" miracle fairy solution everyone here is suggesting, but it is the only one that will work.