Robbie would have arrived to see Professor Braddock, the history teacher, waiting on the roof for anyone who arrived for Wally's suggested martial arts class. Robbie, as usual, was the first student to arrive, but was followed by a few other students, including the girl Wally bodied through the portal at the onset of the attack. Gelignite didn't show despite it seeming likely that he would, whether he's in deeper trouble for blasting that van or brooding over being gassed unconscious, who knows.
Turns out Profession Xavier caught wind of Wally's planned self defense class, incidentally by a professor that could control the wind, seeing as Wally made his grand declaration about it on the roof in front of Professor Munroe. Professor Xavier, rather than blocking something like this, felt it would be better served by providing a professor would some knowledge in the area. It was after all something at least one student had interest in by declaring that he would teach others, despite having no formal education in the subject matter or teaching credentials, but Professor Xavier has always encouraged and supported student interests where reasonable and viable. This happened to be an interest he could support with a professor to teach the students who wished to participate.
"Nice of you to remember your own set appointment mister Iverson."
Professor Braddock looks at the assembled students. "We'll give it a few more minutes just in case anyone else shows up, then we'll move to a more appropriate venue for this kind of class."
That's just in case Ana, Kevin and Brittney are interested. They may have heard through word of mouth about Wally's self defense training from some of the other students in the afternoon or at dinner. They do not HAVE to attend, since we're in some downtime before the main plot kicks up again, we can split the party doing their own things.
Otherwise, including Wally and Robbie, there are about 7 students and Professor Braddock on the roof and won't be more unless Ana, Kevin or Brittney arrive and they'll head down off the roof to the same space they normally use for physical education, outside, but now with some mats laid out