Require an email confirmation at the student email address, say, annually. Also exclude alumni addresses (not too hard to filter 'alum%' or 'post%' and I don't know of schools that use different prefixes for their alumni (speak up if you know; note alum% would get things like alums, alumni, alumnus, alumnae, etc.).
-Sorry for using % as wildcard, but star becomes italics.
-Sorry for using % as wildcard, but star becomes italics.