Not a hobby. Music Composition major, and our child has already composed several pieces (using our PC music software, as well as composing music by long hand).
I would definitely check with the school, because it's probably likely that they have a software package that they use in classroom activities, and I'd wager that software is probably MAC-based.
Only a couple of things to add. My background is in software development on the PC side, but my house has both Mac’s and PC’s.
As mentioned by other posters, what software is in use by everyone else is a critical question. If most of the work is occurring on, or the teacher is more fond of, a particular platform it’s always best to go native. Conversions just add another step in an already long process for an assignment.
If I were buying for a lug around around school device I would seriously look at the Mac Book Air, it’s a very powerful device now, and the SSD(solid state no moving parts to fail) drive are excellent for that application.
Another plus on going to the MAc route is that you can run virtual machine software that would enable your kid to run windows software on the mac. You end up with both worlds available.