Just thought I'd throw this out there since it was driving me nuts.
Had a bunch of Teacher machines not showing any classes in Apple Classroom.
Finally realized that it was because we migrated from an older local server to a new cloud hosted server and none of the pushed apps are actually being managed. So my solve was to make a policy to remove the Classroom app then run a recon which would push it back.
Gabe Shackney
Princeton Public Schools