Isn't it not that likely that this is medical because @joey197860 wasn't warned by the consultants. If the surgery might provoke some kind of endocrine response after a few months, she'd be told. Even if it was 1:million chance, she'd be told.
And because her reaction is to change schools,(to take her D away from the temptation of rubbish food) it suggests she doesn't think it's related to the surgery at all - or a specialist would be her first move.
Considering the 'brutal' discipline at home, I'm wondering if her D has low self worth and is being bullied at school?
Is it an all girls boarding school or mixed?
Is she a weekly boarder?
How does she behave/eat at home?
So many unanswered questions around all of this.
She won't admit being unhappy to her Mum, because she (the mum) likes to think everything it hunky dory - that she's happy, excelling, well adjusted etc.
If this is not medical, it's saying' Help! Why can't you see I'm unhappy.'