Resources in this category:

The Student Center :: http://www.student.com/
"The Student Center is a web community for students; specifically college students, high school students, and teenagers. We try to provide a place where you can find helpful information. But we also want to be the place you turn to for entertainment and fun, so we've got an entire Student Center HUB which is dedicated to communicating with other students."
Youth.net :: http://youth.net/welcome.html
Youth Net is a safe place for school kids from around the world grades k-12 to meet each other online and participate in interactive discussions and projects. There are about 10 projects going on right now, dealing with everything from solar experiments, to gathering memories of the 1940's, to learning more about Charles Dickens through "Victorian Letters and Lives".