Is there a free virtual classroom, and what are the limits of free plans?

Yes, but "free" usually means one of two things, and each has trade-offs.

1. Free plans of hosted services. Many video tools offer a free tier. These are fine for trying a tool or for occasional small sessions, but they usually limit one or more of: session length, number of participants, number of simultaneous rooms, recordings (none, short retention, or no download) and admin features. Free terms also change without much notice. For example, the free BigBlueButton service for Moodle has been reduced over time; see the separate question on that change.

2. Free, open-source software you run yourself. BigBlueButton (LGPL), plugNmeet (MIT) and Jitsi Meet (Apache 2.0) can be downloaded and used without licence fees. The software is free; running it is not. You need a server with enough CPU, memory and bandwidth, a domain and SSL certificate, and someone to install updates, monitor it and fix audio or firewall problems. BigBlueButton 4.0, for example, asks for a dedicated server with 16 GB of RAM and 8 CPU cores as a minimum.

How to choose:

• Occasional sessions with a few people: a free plan may be enough. Check the time and participant limits first.
• Regular classes, recordings you need to keep, or data-protection requirements: compare the real cost of self-hosting (server plus staff time) with a managed service.
• Whatever you pick, test with a realistic class size before the course starts.

Be careful with free tools that do not explain where data is stored or what they do with recordings, especially for minors or regulated training.