After Christmas we audition students on flute, clarinet, trumpet, and trombone. We start with four primary instruments and have them begin there. Student has a form with how they performed on each instrument. It usually takes a week to audition everyone twice and help them make an informed decision. With the student(s) that excelled in our drumming exercises in 4th/5th we start them on mallets with everyone else learning their chosen instrument. I have a chart that has a breakdown of an instrumentation breakdown based on the size of the class. Since we provide all instruments it is easier to say we are having x amount of each instrument for the good of the ensemble.
In sixth grade we move over a few kids to alto, tuba, and baritone if needed. The alto students come in for a short time after school to learn the changes from clarinet. Limited by class size to the ones that are most successful.
I do leave open the possibility of those that want to learn another instrument to come in and learn something else on their own time after school in 7th grade. Almost no one has taken us up on this and completed it. Usually they will work for a couple of weeks and go back to their primary instrument.
They need to complete the entire beginner book, every line we learned in 6th grade, as 7th graders after school to change to that different instrument.