the "old" rule was that once a player steps foot into class, he couldn't be signed.
This was somewhat vague though, because I remember when Kirk Presley of all people was drafted, he was actually on campus for a very short period of time and the Mets got an extension from MLB because Presley played American Legion ball. I was pretty uspet about it at the time.
I'm going to have to check and see if the stepping into class rule is an official rule or if it is an expression. I would have to say that with schools starting at different times, it's probably more of an expression.
It wouldn't be fair if a player at State got a day or two less of negotiating time because their classes started a day or two ahead of Ole Miss. (Just an example)