Each teacher needs to setup a class per period, choose add students and wait for students to join (and then approve). I'd also recommend adding a configuration profile with (see attached) so they can't leave the class without teacher permission.
The old way of doing for teachers is over. They can do it themselves.
I also found that the MDM user on the machine had to match the username of the teacher associated with the class.
In my case all iPads are registered to staff and students by their email address. That all works fine. For a staff member to also be able to use their 10.14 running Mac and get classes automatically through the JSS/ASM their MDM Username (this would be the Account Name under ADVANCED OPTIONS in Users & Groups or RecordName of that person according to dscl) would also have to be their email address. I was allowing my staff to setup their own accounts through DEP when receiving their Macs and since that didn't match what was already setup by their iPad before this didn't work for me either. I changed their account name, reboot couple of times, and the profile does come down.. and it works.