Very good question! It is so important that students of all ages have parent, family, and/or friend's support and interest in their education and learning progress. My daughter-in-law teaches 7th grade and recommends Discover School: http://www.discoveryeducation.com/students/index.c... which has some great resources like interactive games, videos, contests, virtual labs and it also allows students to focus more specifically on one topic as needed.
Another good one is called Eduplace: http://www.eduplace.com/kids/ which prompts users to enter their State and then allows students and/or families to choose a textbook between 5 subject links-
Not to overload you but there is also more than one virtual middle school library out there that can be used as a convenient reference, etc. Here is one that was started by a librarian: http://www.sldirectory.com/studf/stumenu.html#top
Of course one can also google "best sites for middle school students" but you won't get an accurate portrayal of the quality of the material retrieved until researched, so googler beware....
Again I commend you for taking an active educational role. It makes all the difference and sets the stage for a lifetime of good decision making skills.