Watchers in training, for Buffy. One of those ideas that keeps on giving. Like so. (It took two replies for someone to mention TWH...)
It's one of the occasional engines of fantasy settings - where do the sages and wise men and the like learn their lore? We never see Oxford, or the Watchers' Academy, in Buffy but we know Giles arrived full of lore and knowledge (and boyish excitement about getting to use it) so he has a backstory the other regulars don't, and it's a backstory that suggests Miss Cackle's Academy For Witches, Xavier's School For Gifted Youngsters, Unseen University, Jedi Academy and of course nowadays Hogwart's - Wizard School!
I managed to get six years and seven seasons out of the premise - youngish people learning about life as they learn about battling monsters and the like - so I'm not surprised it pops up repeatedly. Here's to the next one!