This is back from the mid 1980's in the UK. The story of a boy who is sent away to a school which turns out to be for witches and warlocks. This was pre-Harry Potter so it wasn't a parody.
The book was quite comedic in tone, and starts with the boy and his awful family deciding to send him away to this boarding school to toughen him up. I remember one of the jokes was that the father or uncle declaring that he needed regular beatings, on the grounds that he'd had them and they'd never done him any harm - before the text revealed he was in a wheelchair.
It was meant to be a comedy, I think; but had some dark themes about the supernatural - our teacher read us the first chapter and then never returned to it. It was certainly more sinister and spooky than Harry Potter or the Worst Witch.
I vaguely remember another male pupil and a female one, and dimly recall the school being on a misty lake. The book had a black and red cover, I think.
Sadly the content mean that it's virtually impossible to google for without being overwhelmed by Hogwarts stuff.