There's two rumors circulating. One is that Walker chose to follow his show Bloody Bloody Andrew Jackson to Broadway, and simply dropped out of the film. The other is that Fox suddenly panicked about his "advanced" age, especially with two "elderly" castmembers like Fassbender and McAvoy. (The truth is probably that neither actor nor studio had come to a decision before the news was reported.) They want someone younger who can appeal to the Twilight demographic, and reportedly tried to lure Taylor Lautner. Given that the wunderwerewolf is super busy, they will apparently be looking for a Lautner-like.
But while one classmate dropped out, another one is rumored to have registered.
Ain't It Cool News hears that Kick-Ass' Aaron Johnson will be playing Scott "Cyclops" Summers. Since the Spider-Man and First Class casting process seems to be overlapping, this wouldn't be a surprise. However, with all the frantic he's-in-she's-out whispers, I won't even bother to speculate on whether he'd be good or not. We'll simply wait and see if it's confirmed, and wonder just how Beast and Cyclops can be the same age in one film, but can leap decades by casting James Marsden and Kelsey Grammer in another. In the meantime, maybe we should hope they decide Fassbender and McAvoy are also too old, and we don't have to worry about them being in a mangled X-Men film.