Don't worry about it. I mean it. It's gonna be hard to believe now, but this is something you will laugh at later.
It will be embarrassing to begin with, but the sooner you can laugh at yourself and own it the better. I know this is far easier to say than to do, but it's the truth.
I don't have any useful suggestions beyond this I'm afraid. It sounds like you have an ally in the popular girl though. Maybe approach her and be open about it? It's likely she will shut down a lot of teasing if she really is on your side and if she isn't then there is no loss in trying. Who knows, you might even make a very good new friend.
I wish I could be more help, but let me assure you it will definitely, 100%, get better. Even if it's hard now.

As far as them sharing it around: I dunno where you live, or how old you are, but it's likely they are committing a crime. Either through distribution of CP, or violation of some kind of "revenge porn" law. The latter is very different from country to country, but generally speaking distribution of nudes without consent is at least actionable. Speak with a teacher about it and your parents.