Hi there, lapdoran!
Thanks for stopping by the Apple Support Communities. I see that you're having trouble using Classroom to view your student's screens. I'm happy to help!
The information outlined in the Apple Support article View and lock students’ screens with Classroom - Apple Support discusses viewing student's screens. It mentions the following:
"Note: Depending on how your mobile device management (MDM) solution is configured, you may not be able to perform this task. Contact your MDM administrator and have them review Classroom view and AirPlay management."
So, the first thing I would suggest doing is reviewing the steps in the article for how to view student's screens, then reaching out your administrator for further assistance.
Cheers!