If you keep those kids out of the drill completely, they don't learn anything about how to march, and then they aren't prepared for the next year. What's worked for me is to separate those students in the drill in such a way that they can march on Friday night, but they don't march at contest. Most drill writers are familiar with this and can easily do it for you. Those kids can have sets in the drill that don't move as much, are easier to clean, and when you get close to contest you don't have to worry too much about it because they won't be there on Saturday. Their parents can still see them March on Friday night. The kids can still feel like they're part of the band.