lucy24 - 11:40 am on Jun 3, 2012 (gmt 0)
Isn't there a built-in php function that does a lot of address-format screening with a couple of simple commands. (For, er, a given definition of "simple". I don't speak php, so the very sight of all those brackets give me the willies.) Though if you're constraining the address to a specific domain, it may actually be easier to write your own. Set it up negatively: if the address isn't in the exact form "firstname.lastname@example.org" then put up an irritating little message.
Oh, and I just remembered that schools always give students an e-mail address. They may not choose to use it for everyday mail, but they do have it available. So that's not a worry.