Cool.  If you are willing to go with 30 years, then Banshee can have been one of the first class, having left shortly after the attack on Shaw.  That would have made him 15-16 at the time.  From there, we can say that Banshee and Magneto fought against each other once, when the second class (I guess it would be the third now) was formed.  That would have been a little over ten years ago.