Ultimately the students decide which solo they will play, but not without some steering from the directors. I give each student 3 choices (starting next week, actually), and over the course of the next month or so we will pull students out in classes to go over the choices. The students will decide which solo they will learn by the end of the 2nd week December so we can get the piano parts out to the accompanists before the Christmas Break.
Years ago I went to requiring solos for all students after hearing Scott Mason talk about it in a round table discussion after winning Honor Band at Coppell. I like the idea of making every student accountable. I know you can do that with ensembles as well, but sometimes it can be difficult to partner up kids due to instrumentation and/or ability levels. At least with solos you know you can easily get something into every student's hand.