Doesn't matter that he was in class at the time of the email transmission... Lots of email programs have automatic settings where you can set it to send/receive all email at a certain (pre-set) time.
All they'd have to do is look at the email headers. From those, they could tell instantly whether the email actually was sent from the computer/dorm room of the student or from somewhere else (such as from the DA's office).
Faking the "FROM:" line, though, is pathetically simple. Anyone with an Internet connection can do it in seconds if they know how. The risk you take is that the headers will give you away, if the recipient is knowledgeable enough to take a look at them.
Why would he warn the others? It's an obvious ruse by the cops. Why would he time-delay the delivery of the message? What would be the point?
Both that player and his lawyer deny he said it.
The cops were trying to shake somebody loose, that's all. When that failed, they strong-armed their way into the dorms and cornered at least one of the players, but none of them talked.