As has been said before, there are explanations.
Maybe Jean wasn't immediately recruited to the school.
And perhaps Xavier didn't gather all the students in one go. Jean could have been among the very first in the movieverse version of events. She could have been there already when Xavier rescued those kids at the end of the Wolverine movie.
Xavier might have had other students for years and years before that rescue took place. Such as Jason Stryker, who it seems was an earlier student for him to then be deep-frozen at Stryker's base on Three Mile Island (I presume Jason had been to Xavier's school already at that point).
Of course, First Class might make it all more structured than that.
It was basically set in stone as soon as it was announced as a prequel.Why should this movie be canon w/ the other films?
I've had enough of Stryker...If they want to squeeze in a cameo, they could, but it's not necessary anymore..
I posted this in another thread, but its more suited for here I think.
---------
I would love the opening scene to be Teen Xavier sat round a radio with his family, listen a news report about Jews being taken into camps (linking to X1 opening scene)
then Xavier suddenly starts picking up the thoughts of the Jewish victims... mainly a scared teenage boy (Eric) maybe into cut with the X1 opening scene.
------------
I think it would be nice to connect the openings. As it is a story about Xavier and Magneto.