SFS Kids is the San Francisco Symphony Orchestra's website for kids. It is loaded with good activities for elementary school and middle school students.
On SFS Kids students can learn to compose and play music. Your students can jump into any section of SFS Kids and start learning, but it is probably best if they work through the sections in order. Students get started on SFS Kids by listening to samples of different styles of music performed by the symphony. A pop-up dialog box appears with each selection of music. In that dialog box students will learn a bit about the style of the composition and performance they're hearing. After listening to a selection of recordings students move on to playing games in which they try to recognize and match rhythms.
In the "performance" section of SFS Kids students learn about the instruments commonly heard in a symphony orchestra. After learning about how the instruments are played it is time for students to jump into the "composition" section of SFS Kids where they'll work through a series of lessons on the basics of composition and begin writing their own pieces.
Applications for Education
Students could spend an hour on SFS Kids and still not run out of things to explore. In a 1:1 setting you could have students create compositions on the site and then share them in your classroom as mini-concert of original compositions.