It's probably more like "you wouldn't know her, she goes to a different school." it's something that's said when someone is pretending to have a girlfriend, often in a school setting. I don't think I've heard it in real life that much, but I've definitely heard it a ton in TV shows and movies.
It's also used a joke when referring to anyone for any reason because the original use sounds really silly even when sincere.
I don’t think I’ve heard it in real life that much
Well, there was that one time a football player from Notre Dame did it, but the jury still seems to be out on whether he was just catfished or was in on the scam.