Honestly I see no alternatives to proprietary soft. Jitsi looks as the best choice, but quality of sound and video are awful there.
Quality of Jitsi calls mostly depend on the bandwidth. Problem here is the teacher requiring everyone to turn their camera on. This will force everyone to download as many videos as there are participants.
I suggest schools to use Owncast or Peertube v3 to live stream. Jitsi can be used during exams when quality isn't necessarily has to be crisp enough to see hairlines but good enough for teachers to see if students are looking away to cheat.
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