Teams meetings are limited to 250 participants at a time. However if your school has an A3 license, you also have access to Live Events. Microsoft Live Events are webinars, where you can stream a single webcam (teacher presenting) or file to up to 10000 viewers.
If you are interested in learning more about live events, refer to this document:
https://docs.microsoft.com/en-us/microsoftteams/teams-live-events/what-are-teams-live-events