Even if you slow down postfix, all those messages are going to get delivered eventually, anyway. The correct solution to this would be to fix the problem at its source--Postfix isn't the problem, your students are. Given that your chances of fixing the students are fairly low (though personally, I would start here with a loss of computer privileges for abuse of the system) I would look to Squirrelmail. Prevent it from sending "hundreds of messages per second" in the first place.