There are so many ways to learn about Astronomy in the K-12 classroom.  

Teaching and Learning About Astronomy.

The concept of Astronomy or Space often begins with young learners learning about their solar system and the history of exploration of "space".  One of the first things that many teachers teach is planet order.
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune

Teaching Planet Order Through Mnemonic Devices
A great mnemonic device for student to memorize the planet order is: My Very Excellent Mother Just Served Us Nachos.  While they might not be able to come up with a better mnemonic device, one strategy for them to learn planet order is for them to create a new mnemonic device for learning planet order.  This Planet Order worksheet is an excellent template for this.   

Construction Paper Planet Order
We also recommend having students make their own construction paper solar system.  This could be done using two black sheets of construction paper horizontally aligned.  Students then are given yellow for the sun, blue and brown to make earth, and random colors for all the other planets that they display and label.  Don't forget the asteroid belt and the planes with rings!

Related Pages
Astronomy Software
Astronomy Clubs