Book about a Magic School set in a castle

Looking for a book that takes place at some kind of magic castle in England. I remember there being something about a magic rock inside of a mirror.

I read this sometime in the early nineties, which is how I know it's not a Harry Potter novel. There was also something about brothers and I remember something at the end of the novel about a magic painting.

The book's cover was green or blue. This was originally read in Russian.