My thoughts on composing - let it happen with NO rules. When students first start to write sentences, we don't tell them the parts of speech. Same with music, let them experiment and offer suggestions.
Compose with noteflight.com
This is an online composing called noteflight (it uses flash, so no iPads) But lots of great features. Students can collaborate online and export as a .wav, .mid, or .xlm. You can also embed the song.
Compose with Musescore
This is a download (cross-platform) and also can be exported as a .midi or .wav file. No embed, since it is an application program.