http://www.discoveryeducation/. com   I think this is a good resource for teachers with many subjects that available.  I would use this site for older students as there is a lot going on which requires the user to go from site to site and may not be appropriate for younger users.  The site has online tutors, homework help on line if wanting a certain discovery channels program could be watched at the same time or individually.  There is a lot of information a teacher could use as a resource site and also refer students to use.