Since the White Queen was the head mistress over Generation X and she is a seasoned instructor now at the Xavier Institute, I would expect her to be played by an older actress like Famke Janssen played Jean Grey. In fact, Janssen played a therapist in Nip/Tuck and I think actress Joely Richardson who plays Julia McNamara, wife of one of the plastic surgens, would be a good choice.
Finola Hughes played her in the ill-fated Fox 1996 TV movie, Generation X.