I noticed that there are some teachers of middle school students. When I taught the solar system in 8th grade it was before state standards and my students came to me with different backgrounds in this topic, some had studied it almost every year and some had no experience at all. To deal with that, I broke my classes up into 10 groups, one for the planets that existed at that time (yes, that long ago!) and the Sun. I had a vertical file of NASA fliers I had picked up at NSTA conferences, books from clearance racks at books stores or checked out from the library, and the computers and I set it up so that they rotated through all the resources to collect information.
Their job was to research their planet/star and the class brainstormed what kinds of information they wanted to learn (climate, atmosphere, distance from sun, moons, size, etc.). They were then to design a lesson to teach the class with graphics and data. They were also to design a quiz to test the class after their presentation on the information they presented. The class was to take notes on a form I gave them (for organization more than any other reason) and to grade the presenters - did they present all the information the class had asked for? was it interesting? did they learn anything?
when they were done, we took the "best of the best" and made a CD for them to keep and donated a copy to a class at one of the feeder elementary schools. So I guess I didn't "teach" the solar system, I let my students do that while I helped. It was actually a lot of fun and they got extra points for creativity. The only problem was keeping them on task on the presentation program so they were putting in information instead of playing with its features and that was a small problem. When classes were not taking notes like they were supposed to, I gave them a quiz of my own over all the planets and they still did better than when I "taught" it after the presenations - they listen better to each other and I made a point of telling them they were guest teachers those days.