J., I think this is all a part of growing up and I am experiencing some of the same with my 12 year old son who is in the seventh grade. He is feeling a "divide" with some of the kids he's been friends with since kindergarten. I do think this transition to becoming a teenager is hard on everyone. I have taken a couple of different approaches to getting him over these bumps. First, I encourage his friends, any and all of them, to hang out here. They all seem to get along better when they know an adult is around. I have also tried to keep my son busy with physical activities and we booked a ski house this winter with the hopes of just removing him from some of the social nonsense that takes place on the weekends.
I think avoiding some of the situations that occur is a reasonable stradegy. I have put my son on allowance so he has responsibilities at home that keep him here vs. with friends.
Long and short, all these kids emerge better people in 9th grade. So I want my son busy, tired and at home as much as possible til then. That said, what will make our kids most "successful" is encouraging right choices vs. "popular" choices or going along with the crowd, even if it results in some lost fun.
As far as discussing with the other parents, it is hard to weigh in on that not knowing the parties. Some parents are receptive and some could care less. I would feel them out first before you confide in them.
I completely understand your upset. No easy answers anymore, good luck, A.