Hi,
I'm sorry but there is no way to inhibit users from unmuting themselves.
It's also not possible in Teams to unmute someone else.
Searching for this feature in Teams for Education User Voice, this feature seems to be under development:
https://microsoftteams.uservoice.com/forums/599053-schools-and-universities/suggestions/39949276-hard-mute-students-audio-in-a-meeting-so-they-can
Stay tuned!
Regards
Luca