The simplest solution is to use their student ID or student information system identifier (numeric usually and not their actual student ID).
Example: student_id@domain.tld, with the display name assigned to be the student's name.
But Simple is probably not in the vocabulary of a university that let their address space balloon to 350 Domains in the first place.
Seriously, how does something like that happen?
But you have to ask, why bother setting this up at all?
EVERY student entering college these days already has an email address (or maybe 3 or 8).
It would seem the only people that benefit from a numerical student ID based address would be the school administration, so that they could send email to all students without having to think, or even bother to look up the IDs in the on line records. Yet college administration are the one group most capable of looking up everyone in an on-line database, so why do they need a numerical ID?
Just use the student's existing Email address.
In the mean time, consider that not every student WANTS a publicly known or guessable email address.
Most students don't know, and probably shouldn't have to know anyone else's student ID number.
There are benefits to the school of NOT maintaining a huge mail server when other sources will do this for free. First of course is the cost, second the policing that is required.