I was just trying out ScienceSim, an OpenSimulator project sponsored by Intel and run and maintained by a loosely organized group of participating scientists and programmers. The main goal of the project is to "Enable new usages in education and visualization through the construction of persistent 3D spaces build and deployed by a federation of organizations and users (from the website)." You can create a free account at the first link and follow the instructions for downloading a viewer and configuring it to connect to the sim. Then you just explore. There doesn't seem to be any guiding organization to what projects and simulations are developed there and some could really use more information to be useful to the outside observer. One, however, is far and above the most worthwhile, especially for a teacher, and worth the price of admission itself (which is free, but...). That is the planetarium, which allows you to fly your avatar among the planets as they orbit the sun. UPDATE 1-27: To get there go to the Pascal region. The easiest way is to choose Pascal from the region menu on the left of the login screen. Or you can teleport over using the map once you log in. It would be an excellent simulation for any elementary or middle school class to experience as you can really see the different rates of the planetary orbits.
Some other simulations are promised but not always accessible. The project is comprised of different people hosting their own sims and linking them to this main one. So if someone's sim isn't running at the moment their teleport link won't work.