I just want to know if anyone knows of any of these. I've seen Mario Paint and Sim Tunes but they don't seem to actually teach anything, just be toys. Also I would hope a good educational music composing game would be able to export the student's compositions as sheet music.

I don't know of any either, but it's actually something I've thought about quite a lot recently, as I think it's a type of educational game that could really do a great job of actually being genuinely fun and genuinely educational. Aural skills (interval training, rhythmic training, etc.) would also be rich areas to explore.

So... yeah, I'm also interested to hear if anyone knows of any games of this sort!

Darn, did I forget to respond to this earlier? @tonemgub thank you for recommending that game, not only was it fun but it gave me some new ideas about how a music composing program that could fuction as both a toy and a tool might work. :)