What is a teacher to do when he has to be gone and the sub knows nothing about music! Have the students lead the rehearsal...after all, they probably know more than the sub.
A student led rehearsal is different than having a student conduct. Nobody conducts in a student led rehearsal. The students collaborate.
I stole this idea from Dr. Menghini of Vandercook College of Music, who presented a session using his own students. I have used it with much success almost every time I have a sub.
Almost Everything I've Learned About Teaching Band