I feel for you Op. My DD invited around 6 girls (mainly new friends) for a birthday meal in yR 7, all initially accepted.
One girl (a friend all way through nursery/primary, but drifted apart in Yr 7) decided she wasnt coming (or maybe we hadnt invited her, cant remember actual detail) and proceeded to tell all others that they shouldn't go either!! So 4 girls actually recinded their acceptances just before the meal. It was awful, I really felt for DD.
But, we went with just the other 2, and had a lovely time.
She is now Yr12, almost 17, and has moved to college along with 2 friends who she became close to in Yr 10/11. The 3 went to Prom together, a summer festival and now travel to and socialise at and outside college together. She does see other school friends, who stayed at school or went to different colleges, but I would say that, of the girls she hung around with in Yr 7, only one girl is still a friend.
A friend whose DD is same age as mine, told me that her DD was excluded and treated quite nastily in Yr 8/9, by her so called friendship group. Girls can be nasty.
And YY to mixed groups. DS was never a lads lad, hated sport, always more friendly with girls through primary. He did have some male friends in early years at sec, but always quite fluid. By Yr 10 he was firmly ensconced in a mixed group, including some of his primary female friends. That group did Prom together, socialised all through 6th form, and are still friendly now at uni, eg travelling to each others uni for birthday celebrations.
He has settled really well at uni, has a mixed group of male and female friends, some of whom he met on day 1, and is thoroughly enjoying himself.