One of the biggest problems that any percussion director faces, besides getting instruments in one place, is that at times, students can't be at every rehearsal. I have found that this is really an excellent opportunity for a student who is not in a piece to get an opportunity to read a part that they are not necessarily going to play. Now you are combining real life experience and solving your momentary personnel problem all at the same time. Of course, wholesale and frequent absences from any type of rehearsal can not possibly work, but on the occasion that you have to deal with this problem, solve it with an unexpected experience for a student. Try not to focus too much on the substitute player, if possible. It is an extremely difficult situation to be in and one that can be thankless if the substitute is treated as though they should be as up to date as a regular ensemble member. Be gentle to the substitutes!