I wonder if anyone's done an academic study into the dynamics of girls' friendship groups?
Picking up on your points, I think one reason for your different experience might be the huge divide between pre- and post-80s Britain. Even the poshest girls in my school only had possibly 2 outfits other than their school uniform. I had my 'best' clothes for church and maybe one precious item bought from either M & S or wow, Chelsea Girl, which had a branch in the small industrial town. No online shopping, we never went to London etc etc, so there really was no opportunity for the designer label status culture which came in in the 80s. There was no reason for clothes snobbery (on school trips - we had a very strict uniform otherwise) at all.
I'm sorry if this sounds like the Monty Python Yorkshiremen sketch!
Also, your school was co-ed. I'm not making a simple-minded argument against co-ed but in both my single-sex school and my son's and daughter's single sex schools, the absence of the male gaze (and the female gaze!) took a lot of pressure off. I know, because I heard of the experiences of my friends whose children had become very aware of sexual politics/attraction/pretty privilege etc pretty much from Year 7. I may get flamed for saying it, but where the opposite sex is present, the Darwinian struggles for approval and status are definitely activated.
Bullying - it was mild, in my experience. I didn't have the local accent AND i had a stutter when I went to grammar school, so did get teased and called 'posh'. I knew I wasn't posh and I didn't much care about that jibe. Those girls changed and became friends eventually. There was one girl who would beat people up, but she was a rare bird in my school.
I also think that because it was a northern industrial town we were a fairly unsophisticated (some would have said 'provincial') bunch and were innocent of the kinds of snobbery and sneering which might have been prevalent in the more affluent areas of SE England, for example.
I saw no point in feminism at school. I didn't get it because I'd never experienced it, with all girl schoolmates and strong women teachers.
Oh boy, when I got to university and out into the world....THEN I got it.
But I still think the imported High School/Prom culture imported from the US is a factor in making school group dynamics and women's relationships in general more toxic.