It's one of the versions of the line from The Breakfast Club where Brian claimed he was dating a girl from Canada.
https://m.youtube.com/watch?v=K9UxiynPxI8
The implication is always that she doesn't exist, and often that the guy is a virgin.
Ironically, I actually had a girlfriend that went to another school during my final semester of high school, and I was basically Brian.
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Avenue Q did it also.
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.
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