I would kind of like for the teachers and staff to be canon characters, but if we want to avoid those all together I'm perfectly fine with OCs in those roles. I do, however, agree that Xavier should be our headmaster.
I've said this before about another X-Men RP, but with 50 years of continuity to consider it's impractical if not next to impossible to make an RP fit in perfectly. My suggestion is that we play this loose and just use the core themes and elements that make the franchise what it is: A school for mutants where they learn how to control and use their powers for the greater good. It can be its own universe that borrows from the comics, movies, and TV shows but isn't beholden to any of those timelines.