Not just in contemporary fantasy, a la Harry Potter, where if the wizards are spread thinly, you need to have a boarding school to get 'em all together. (Or equivalent thereof -- summer camp in Percy Jackson, an after-school program (and one that takes up an inordinate amount of time) in Ever After (and there she ended up boarding in the end)). In such high fantasy as Ursula K. LeGuin's Wizard of Earthsea or Patricia McKillip's Riddle of the Stars.
To be sure, those are pseudo-medieval settings, and part of that is that most folks don't go to school. Any school at all is a prestigious thing, and one that does formidable magic is a high prestigious thing. Even in Victorian times, schools close enough for most pupils to walk to would probably not go into any magic, except the routine. Patricia C. Wrede's Frontier Magic series does have magic lessons in a day school. Though considering the size of the town it's in, and that it's a university town, that may be above the usual standard.
A magitech world, up to comtemporary tech with magic, would probably be the best bet for a magic school. Though if they wanted to concentrate, it might still be a boarding school. At the very least, it's probably a magnet school. Hmm. Operation Chaos doesn't get into the details, because the school story is a college one. (And our narrator an older than usual student.)
Then boarding schools have their advantages in putting the characters together in a fixed setting with minimal departures and arrivials. That's a desirable quality in a setting.
