It doesn't solve the problem of dishonesty or possible inequities, but it gives you some type of documentation. I often drop the PR grade if they are playing up to standard and doing well in class if it potentially lowers the grade. If they are not progressing or participating well in class, but PRs look great, then you have something to back up discussions with parents - either their kid is lying, there is a major problem with HOW they are practicing, or maybe band isn't for them. In that case, it's weighted so that they're only making a B or C even with 100s on PRs. On the other hand, if the kid is truly practicing and participating but just playing very poorly, you have a justification for giving them something better than a C or F.