I mean, it's understandable why MC and Amber would hate them based on what was shown in the first episode and most likely Tommy also experienced something similar. Bullying like that can be traumatic and leave long lasting scars and hatred, even if they stopped behaving like bitches they were in highschool, they haven't done anything to make amends, even going so far as to belittling and being condescending toward those they hurt and completely disregarding their feelings as anything that concerns them. So it's not like MC, Amber and Tommy have any reason to not hate them with the same burning passion as they did in highschool, considering it hasn't even been a year since high school ended.
Only ones that have even made slight attemptions to have friendlier relationship with MC are Annie and Sarah, and even they are still being condescending about the pain MC had to go through because of the other members of bitch squad. And while neither of them were shown to be actively part of the bullying/harrasment other members of the bitch squad did, they did give it implicit acceptance by not speaking out against it. Sad fact about bullying is, that it's never just between the bully and victim, others around either fuel it on by encouraging the bullies, or just being silent and accept that victims are tormented.