Ultimately you may not be able to convince parents that band takes priority sometimes. There are a lot of selfish people and parents out there.
You may, when fighting this, lose a few kids in the long run. But it is a cultural thing. As soon as kids and parents understand the culture of expectation then it's going to disappear.
Alternatively you can avoid scheduling things on the open week and weekend.
Someone once told me "When fishing for the bad fish you may catch a few dolphins." It may suck in the short term to lose 2-3 kids because you're putting your foot down, but in a few years the issue will go away.
Just do it gently. It's not YOUR decision, it's band policy. "I'm sorry Mrs. Susie's mom, but band handbook states that if a child misses a competition then they lose their spot. I'm sorry it's happening to your daughter, I feel for her, but this is the policy I am expected to maintain."