After trying out a few solutions, I found <a href="https://bigbluebutton.org/" rel="nofollow">https://bigbluebutton.org/</a> by far the best for classroom settings or even for general meetings. Being able to upload a presentation as a collaborative white board is much better than simple screen sharing (which is also possible). Video quality is not the best, but audio is rock solid and we had zero connection problems with tens of people in a conference. Getting started is simpler than Zoom, since no installation is required and both Firefox and Chrome are fully supported. It is also pretty easy to self-host and there is even a load balancer for large installations: <a href="https://github.com/ICTO/BBB-Load-Balancer" rel="nofollow">https://github.com/ICTO/BBB-Load-Balancer</a>