Is the student's authentication account not being disabled after leaving the district? That would seem to solve the problem, as they would no longer be able to use it for DEP enrollment unless they knew someone else's account. They could wipe it all they want but would never be able to get farther than the enrollment screen. It also sounds like you're relying on smart groups to restrict the iPad. If you scope a restriction profile directly to the username/device, it should receive it immediately on enrollment and remove their ability to install/use apps before they get a chance to put anything on it.
Otherwise, my suggestion would be to make a separate additional restriction profile with the "Allow Erase All Content and Settings" option disabled and deploy that specifically to the student's iPad. I don't believe that would prevent them from placing the iPad in Recovery Mode and wiping it through iTunes however.
If you know the student's AppleID password, you could change their password. Then if they do try to wipe it through iTunes, they'd get stuck at Activation Lock.