Whyville

Whyville (not to be confused with Whoville) is a new preteen/teen simulation world for ages 10-16. You can create an avatar, chat, build, and buy things in their currency of clams. It is not quite at the level of Second Life, but after much exploration, I think it would be much easier to monitor in a classroom. There are City Workers (CW) who monitor the language, content and other actions in the world. Almost every world I visited had some educational value or task. Trips to the mall are turned into math games and going to a restaurant is turned into a nutrition lesson. It educational resources already built in The interface is a little simplistic, but for classroom use, this is a great tool.

Check it out- Whyville