Grammaretto
I didn't survive until the 6th form at the horrible school Cariadagain though I was told by friends who stuck it out that it improved as yours did.
I was sent to boarding school for my 6th form. I much preferred it. No bullying as far as I was aware.
Sadly my own DC were bullied at school too and some of my DGC are not happy there. 16yr old DGD has left and is going to football college.
That's sorta confirming my view why bullies are that way - ie things improved come the 6th form (ie they had left) in the school you're referring to as well.
It was quite a liberal/modern school all round and did have a lot going for it in many respects and I do wonder what would have happened if I'd turned round to teachers there and told them what the bullies were like and generally "naming and shaming". I guess I have to forgive myself for not having the sense to do what modern-day me would do without hesitation, ie keep a record of times/days/events, suss out a suitable teacher and tell them what was happening and demand the bullies were dealt with. If they weren't = I'd be off to the local paper to report the teachers for not protecting me from the thicker pupils.
I am pretty convinced the bullies are that way because they know, at some level, that their victims stand a better chance than they do and are trying to pull them down to their level. The first problem was I had simply no idea they werent at the same level at the time - and only began to realise very late on - ie when the University sent in people doing IQ tests on us and I went "What does a score of 130 mean please?". If only only they'd done that test on us all years before they did....and I might have realised what was what and reported the bullies....
I could also have done with a Sociology O level being available at the school - perhaps as an "added voluntary extra" - as I subsequently decided years later to do one at evening class out of interest and that was quite a revelation as well and helped me realise what had happened - as I analysed what had been going on there. Yep....even an 0 level in that subject is very useful imo.
Bullies - of any description - tend to regret it (a lot!) if they start in on me these days - as I've learnt a lot about how to deal with them......record-keeping, official reporting, reasonable knowledge of any relevant laws and totally openly letting everyone know what's happening. I've learnt that - if a bully starts - it's NOT MY FAULT....it's a character defect of theirs.
Perhaps those thoughts might help your young relatives deal with these characters?

