Fox has already set a release date of June 3, 2011 and plans to begin shooting this summer. No cast has yet been announced, but maybe you can go through Vaughn's filmography for some suggestions. Surely Chloe Moretz can play a young X-Girl? A young Jean Gray, perhaps?
The film will tell the origins of the X-Men that you've already met, like Cyclops, Jean Gray, Beast, Storm, and gosh knows who else.* The comic featured Iceman and Angel among the young wannabes, but as Fox wrote those into X3: The Last Stand, don't expect them to make an appearance. Instead, Magneto and Professor X will probably take up the screen time, as reportedly Magneto's origin film has been folded into this. And I'm sure they'll find a way to work in a Hugh Jackman cameo. Maybe he'll drive by the school on a motorcycle, or be their groundskeeper for a day. He won't remember it, no one will remember him, but it's not like continuity is Fox's strong suit. Superheroes are Vaughn's though, and I certainly hope he can fight the good fight here.
*ETA: Apparently, First Class won't be about those young X-Men after all. Variety is now saying that the film focuses on Charles Xavier and Erik Lensherr as they discover their powers, take on their aliases of Professor X and Magneto, and work with other mutants "some familiar, some new" to stop a global threat. Who will be those new or familiar mutants besides Wolverine? Feel free to guess!