This site isn’t directed to K-12 students or teachers in particular, but it’s the best source on the web for information and photographs of all the planets in our solar system (and their moons), plus the sun, asteroids, comets, and meteorites. You can even choose among English, Spanish, or Portuguese text when exploring the site.
Hundreds of high-quality computer images include recent Pathfinder photographs of Mars. Several animations also spice up this site, such as 360-degree animations of volcano calderas and a 360-degree illumination of the “Face on Mars.”
The accompanying text for each planet gives a history of its exploration, describes its characteristics, and details recent discoveries.