I'd stick with the 'all magical kids go into the woods and only the girls come out again' but I'd come up with something less grimdark than the obvious for the boys. I'm a fan of the idea of Brettonia as the land of polite fictions, where, for example, it's true that women can't be knights, but women dress up as men to be knights anyway all the time and nobody says anything about the beardless lad with the high voice who's oddly private about bathing, and if the truth does come out in a way that nobody can pretend it didn't, then the 'punishment' is to Seek the Grail which gives them knightly status anyway.