An app and a site

Technology can help you in the classroom. But as far as Garage Band goes there are limited uses.

When explaining the difference between a staff and a keyboard I would open Apple's Garage Band and display a keyboard. You find it under the Windows menu. Then size it very large so students see a keyboard. It helps in explaining steps, accidentals and especially scales.

I would also at the same time quickly go through lessons 1-5 & 8-9 at 8notes.com/theory. 

Both tools present easy to understand concepts that students at any age can grasp quickly.

Occasionally I would see lesson plans offered to teachers about classes using Garage Band to write and/or record songs. You would have more success teaching them a three-man-weave on a basketball court. Rather, we would make up songs and NOTATE them on the whiteboard. Teaching students to randomly press buttons to be "creative" is the bane of our modern music culture. Make the music yourselves. Don't allow the robots to do everything for you!