Sorry guys. Yes the invitations are in Self Service Portal under notifications. Says I need to accept the invitation to receive JAMF MDM... or similar. Which I enter in our JAMF Apple ID into and all is set. But I am wondering if this is going out to all students, they just do not see it.
We use VPP stuff pretty heavily but mainly device based assignments. Students still log into their iPad with their Managed Apple ID though. I have not seen an invitation go out through the method you are mentioning but it might be because we have Jamf auto accept those invitations.
Settings>Global Management>Volume Purchasing>Click on your account you might have setup>Automatically Register with Volume Purchasing if users have Managed Apple IDs.
I am not sure this setting is what is causing it. Our students register the devices during enrollment with their AD accounts and then it links with their ASM account and does everything else on the backend.