Is there a virtual classroom platform?

I've played around with Big Blue Button in the past. It's open-source web-based designed for distance learning (although I was testing for my company). It has video, whiteboard, desktop sharing, audio, chat and is under pretty steady development.
The problems I had were mostly bandwidth. You can build and deploy your own server but you'll need a good chunk of bandwidth to make it work well with lots of participants. Also, if you don't have a static IP you'll run into issues with the way the system works. It can run with Dynamic IP as I recall, but it takes some juggling.

Edit: It also has no front end, you'll need Moodle or something like it to set up meetings and invite participants.