We're just talking about software delivery here. Its the same as Debian not requiring you register before using `apt` to install packages (or every other linux distro).
Your argument boils down to "I came up with one scenario where this is bad, so it can't work at all" and I find this dissatisfying. If this hypothetical student "shared" their user account and then disavowed giving it out, you would have the same issues.
You can easily generate individual share links for every pupil and sanction the one whose link was used by a hundred random people from all over the world to join the conference. Jitsi and Big Blue Button are both able to handle this special use case where users aren't trusted to act in good faith I believe.