The CDC http://www.cdc.gov/ This is the Center for Disease Control and Prevention where students can learn about diseases and how to keep healthy.
PBS Kids
http://pbskids.org/ PBS Kids
For the younger audiences out there, this sites offers learning in a fun way for the young-lings to learn and develop.
Khan academy
https://www.khanacademy.org/
Khan Academy is a fun interactive learning and education web site where students of all ages can explore a vast variety of subjects, master them, and continue learning. Greatly recommended for teachers, students, and even parents.