I've worked for and with Mean Girls. They're awful, and they never change. You have to think of it in terms of bullying, not just as a high school situation. There are legal resources available for your employer to demote, retrain or remove bullies. Go over their head, don't retaliate, and document everything. You'll be respected for standing up for yourself, and will probably make some strong allies at work who have also been victims. One person just has to be strong enough to stand up and say STOP