Currently, when you are creating an assignment, the only two options you have for configuring the instrument for your students are 'Let student choose' or 'Rhythm only'. The main reason for this is because certain instruments and levels aren't compatible with certain key or time signatures. And if you picked a combination that wasn't legal, music would not be able to be generated. With that said, we do hope add some validation to that page so that it will be possible to set the instrument(s) for an assignment.
However, you can essentially set the instrument(s) for an assignment by using the Custom Levels feature. If you create a Custom Level (in doing so you will configure certain instruments for that Level) and then set that Custom Level for the assignment, when your students launch the assignment that is using your Custom Level, their instrument choices will be limited to only those instruments that you have configured for that Custom Level. So, for example, if you created a Custom Level that was only configured for the Soprano voice, when your students launch an assignment that uses that Custom Level, Soprano will actually be automatically picked for them. Check out this page for more information on Custom Levels.