Then, you have two examples to share with them depending on whether they are academic or sports minded. For the academics use an example such as walking into a classroom and randomly picking 10 kids to take the test and 10 kids to sit out but the score goes to all 20 kids. It's pretty obvious to see how that doesn't pass the logic test. For the sports minded the example is fun. The defense is playing with 11 players. But your offense, which had to leave kids behind, puts a quarter back on the field but no center to snap the ball and no receivers because they had to stay on the sideline for the series. So the QB has to take the snap himself with the entire defense able to rush because all he can do is run or give to the running back to run and thus they are toast. That also doesn't pass the logic test.
The band is evaluated based on the entire ensemble playing. Band isn't an individual activity, (That's solo and ensemble), but requires ALL participants. Just like the offense can't play without an offensive line the band can't play with half the clarinet section sitting at home. Every kid takes the STAAR test and the school gets a rating based on the average score. Same with band.