I can't stand people like that, it's pathetic that people go around making other people's lives miserable, just for kicks.
Although I say that, I'm pretty sure I read a news article recently which stated that some bullies have problems at home which is why they treat other people the way the do.
I think others just do it for kicks though...

Ignore them, move on with your life. Them picking on you because you left their group prooves that they weren't very good people to be hanging around to begin with.
True friends will stay by your side and support you, no matter what, and back you up when things get tough.

If you start to feel that it's getting out of hand however, then talk to your parents or a school councilor, don't keep it to yourself.
No-one deserves to be treated like that.

Once they've all graduated, and actually start having a taste of the real world, they'll have a very rude awakening. If you want to get far in life you can't go around treating others like crap.
You probably wont have to see them again either, which is a bonus.

For now though, delete all their numbers, emails etc....but I would suggest keeping any nasty texts you recieve and send to show to your parents, school councilor or whoever you decide to talk to about this. This will proove as eveidence that they're bullying you.