Suggestions on How to have a virtual pep rally

Hi howrch.  My instinct would be to have everyone in one big session.  It sounds like you essentially trying to have a meeting inside another meeting? You have the pep rally meeting, and then each class is participating in the pep rally but also together as a class in a class meeting? (if we were together, this might be analogous to a class sitting together on the bleachers in a gym).  

Untested Brainstorming:

1. Schedule Teams Meeting for Pep Rally
2. Have Teams Meeting for each class
3. Teacher joins the Pep Rally signed into Teams in their Browser, NOT the teams Mac/Windows app
4. Teacher joins the Class meeting in Teams app
5. Teacher shares browser, with audio, into class meeting. 

Considerations

• Probably too many moving parts - the technology would work but it's really unlikely the humans would work! I suspect a LOT of faculty would get very confused on the concept of a nested meeting, and the teachers would have to pay so much attention to what meeting they're joining and which teams instance is using which audio device, I think it'd break
• The pep rally would be "one way" for the class students, they're just viewing what the teacher sees
• The teacher could curate feedback, reactions from the class, post into the Pep Rally session

If you did the Pep Rally as a Live Event, instead of Teams Meetings, that might simplify things a little, but you should schedule it as a Live Event in Stream so it renders in a browser, not the Teams client, to avoid that double-joining risk.  However users tend to stumble the first time they share sound from a computer as Teams and your computer need to be set to the same speaker.

I'd heavily consider doing rally as a single event (Live Event), all students in as attendees, and not try to curate it into classrooms, due to the reliance on every teacher needing to 'join and share content the right way' and the likelihood that might go awry.