If you find a set of links you're interested in, you've found a goldmine. Over 1000 sites cataloged, described, and rated. From New Zealand. Link Library from Houghton Mifflin. Some links at top are specific to the publisher's textbooks.
Many links have general relavence.
Encarta Schoolhouse - Selected Education Sites General directories Web Sites for Students Categories for elementary and middle school grades and high school subjects. The Awesome Library Links to 15,000 web sites. Directories and Resources from the Open Directory Project. See also, subject directories which are massive but very general and the K-12 directory at this site. An encyclopedic site that cannot be faulted for lack of ambition.