The U.S. Geological Survey USGS), within the U.S. Department of Interior, provides a tremendous amount of  online educational information for kids.  When first entering the USGS Educational site, at the top of the middle column titled, "Educational Resources", there is a link to resources for grades K - 6, and under that link a second one to resources for grades 7 - 12.  Within these two links there is information about maps, ecology, caves, rocks, earthquakes, disaster preparedness, glossaries, and more.  With the information comes not only interaction tools such as games and puzzles, but also classroom plans for the teacher.  The main education page offers other links for any age level, but these two mentioned are specifically created for K - 12.

http://education.usgs.gov