I think it'd be cool to have her in first class as one of the original Xavier students, and then show that she left/graduated/whatever just like Beast did. If/when we get a true X4, show that after the Professor's death she decided to come back and help run/teach at the school. Might need a different more mature looking actress for X4, but I think the story works. Actually, you could do that with alot of X-men who haven't been in films yet. Just say they were part of the first class and moved on, but return in X4 to help fill the gap left by the deaths of Jean, Scott, and Professor X. In the novel version of X3, I'm pretty sure it says that Bishop was one of the original students too.