Apple Classroom: Offline Error

Teachers are setting up classes (Ad Hoc, not Managed), and students are all able to join the class. This works for many students. For some students, they show up in the class, but show Offline no matter what their activity.

We've tried:

• Updating iOS (all devices experiencing problems are on 12.1.4, as is the teacher device)
• Restarting the device
• Turning Bluetooth off and back on
• Turning WiFi off and back on (and checking they are on the correct WiFi network)
• Checking WAPs to be sure they're on the same access point as the teacher. (Nearby WAPs work, too - students whose activity is showing are on either.)
• Removing the class on the student device and re-adding.
• Ending the class on the teacher device and restarting the class.
• Checking the MDM (we use Airwatch) to be sure the device is communicating with the MDM and with Apple.

In one class, we got an error on the teacher device in Classroom, saying that the three students [whose activity was showing Offline] hadn't joined the class in a year, and to remove those students. There was no option other than to Remove them. That did not remove the class on the students' devices. It also did not resolve the problem after students re-added the class. We did NOT get that error for two other classes who also had Offline students.

We are trying to encourage all teachers to use Apple Classroom. Teachers are frustrated by students' activity being "invisible" to them.

Thank you for your help!!