I am an over prepper, so I type everything out and leave it for them. What we play, what tempo, etc. But more importantly I have them do it a lot of times prior to me being gone. I usually pick two kids to take turns in case one is absent, but also just to give more than one person some responsibility. I guide them through what we are doing, and the type of things that should make them stop and start over. I also have another student who writes down my attendance for me on most days, just so I can take care of it after class and not waste time during class. This also helps the substitute. Oh, and they know that one of them will always email me to tell me if anyone was being a pain. I always get great comments from my substitutes when I am gone. And I figure whatever they played was better than nothing.