Jim Heynderickx has been instrumental in bringing virtual worlds to the attention of the independent school community. Some of his thoughts on the subject can be found on his blog.
Second Life is a very popular 3D world.
There is a Second Life for Teens which seems to be making an effort to reach out to parents and educators to help in creating a safe virtual environment.
For immersive learning in a 3D environment, reserach based - Quest Atlantis - for 9-12 year olds - from Indiana University