I don't think that was "illegal" any more. You have been already pre-accepted to the UofC and you have a student id, so it might have been perfectly legal ;-)
In my mind, at least, "illegal" would be bouldering on the side of a building; bouldering in the UC rock gym (if it exists) I'd think would be "legal"...
The wall is free for UofC students and Michael is still at the High School. However, he was pre-accepted at UofC. Consequently it is this "gray area" where he should not be there because he is not studying at UofC, but technically he is a student starting in September. So one could argue both ways. That's what the joke was about.