I think that was the idea? He may not have stuck around long enough to actually get a class, but I'm pretty sure he's in the employ of the school. And they're in New York. I have no idea what the age of consent is in New York, but I know in at least one situation someone over the age of 21 had to double-check that the girl he was hitting on was as well, or at least over 18. It was a fictional situation, but based on IRL laws. LOOK I'M JUST POINTING OUT THE PRACTICAL ANSWER TO YOUR QUESTION. YOU WANTED TO KNOW WHY, SO I TOLD YOU. And in every other incarnation Rogue got her powers when she was 13, so just be glad they aged her up at all.