After reading as many Silver Age comics as I have lately, I could probably make this a weekly series in and of itself, but this one I read just a few hours ago, and it's the sort of thing that sticks in your head. (An appropriate phrase, as you'll soon see.)
In the original 'X-Men' #1, by Stan Lee and Jack Kirby, we open with a sequence of Professor X running the team through their training regimen. Beast has to do a difficult acrobatic routine, Angel must fly an obstacle course...then the young sixteen-year old Iceman gets a turn, but Professor X is "going easy" on him by merely requiring him to display his powers. Iceman frosts himself over with snow...and Professor X telepathically tells the Beast to chuck a bowling ball at his head while he's distracted, to "test his reflexes".
Three thoughts on that:
1) Best argument against vouchers EVER.
2) Anyone who doubts Xavier could ever become Onslaught, this is Exhibit A for the prosecution.
3) I think we know why the students all wanted a Danger Room. It was that, or Xavier just had them whip heavy objects at each other.