Hey Seth - if you're a Chromebook or MacBook school, we've found this chrome extension to work to keep students from joining a Google Meet until the teacher has started the Meet. Doesn't matter if the Meet was created in Calendar, Classroom or Meet itself - the extension will only let the creator of the Meet session start the Meet and then others may join. We don't allow our students to start their own Meets either. So far so good! Teachers may also want to turn off Quick Access - that could help on it's own. Google is starting to roll out the ability to turn off Quick Access when creating a Google Meet from Calendar this morning. They also recently announced sticky settings where once Quick Access is turned off for a recurring or Nicknamed Meet it will stay off.

(One thing to note around the extension method I linked above- we only enable Chrome as the browser on student MacBooks (Safari is disabled) so this works for our 7/8 and our 3-6 are on Chromebooks so this works for us. If your students are on iPads or use Safari, this method won't work.)