not really sure what actually this casting thread is for...(the "first class" prequel, a X4 sequel, or a reboot???)
but, here are some thoughts I had on a prequel
Scott -
Jon Foster

Jean -
Mary Elizabeth Winstead

Storm -
Lauren London
keep...
Xavier
Beast
Magneto
...the same cast
(just de-age them abit with make up, or the CG thing they did in X3, an give Grammer a wig)
Gambit-
Michael Trucco

since Gambit, is in the Wolverine movie, (which is set like in the 70s??) and Gambits like 20's-ish, so, he would be like late 30s in this timeline.
I was thinking maybe he could be like there as a member of an earlier, x team (even though, that ruins the mytho of the comics original team) thats made up of him, Beast, Xavier, Magneto, and maybe, Emma Frost (oh, also, possibility, Forge, it would help explain how they made all the tech stuff)
and they can all make up the faculty of the school, teaching the young recruits
Emma Frost -
Natasha Henstridge

(Forge - ????)