Starchild.com offers students in the 1st-6th grade crowd a great resource for learning more about our solar system and universe. It comes complete with activities and a glossary of space terms. It also offers a text reader for those students not yet reading to read the text. There are 2 levels: level one is great for grades 1-3/4, and level 2 is great for those more advanced. There are even printable activities for your class.
StarChild has a sister sight for older students. Imagine the Universe is designed with the 7th-12th grade crowd. It offers more advanced explanations of all things space related. It has a user interface that is more similar to a blog than that of StarChild. It also gives resources for additional resource. It too comes with a teacher resource page.