The following are teacher/Principal approved websites, suitable for High School students:

A Plus Math
Art Made Easy
Arttalk/Glencoe
Belle Plaine Public Schools!
Census.gov
CIA.gov
Classzone
Cool Math
Dictionary.com
Discovery Education
Education2020
Engrade.com
FactMonster
FoxNews.com
Funbrain
Glencoe.com
HistoryMatters.com
MSN.com
MSNBC.com
NASDAQ
National Geographic
NiceNet.org
Nova.PBS
Online Conversion
PBS
Read Write Think
SAT.Collegeboard
Science Made Simple
Spanish Dictionary
Spanish Spanish
Study Island
The White House
Wikipedia.org