An Interest of Conflicts

A student approached me today and said that her softball coach told them to always put softball first. He said that if they are involved in other things, softball should take first priority. She questioned him saying, "but we have choir practices, mostly in the mornings, but sometimes after school." He replied with "Mr. Cater and I are like this (fingers crossed in the air), I can talk to him. We are fraternity brothers." Now, this is true. The middle school softball coach, who teaches at the high school, is one of my fraternity brothers. That's not what I want to discuss. It got me thinking about conflicts between extra curricular activities. It happens to every choir teacher. I want to have one after school rehearsal, and fifteen kids say they can't come because of soccer, basketball, cheerleading, wrestling, etc.
I'm still working these things out in my mind, but shouldn't there be some sort of universal rule about this? I say yes. Here's my crack at said universal rule:
A game takes precedence over a practice. If I want to have a choir practice, and a student has a soccer GAME, the student should go to the game. Likewise, if I've got a concert on a day when a student has a cheerleading PRACTICE, I get the student at my concert. The trouble comes when I need a student for a practice, and she's got another practice. Here, I defer to the frequency and importance of the practice. These two variables have an indirect relationship, meaning that the less frequent a practice occurs, the more important that practice is. If she has softball practice every day, and I only ask for one extracurricular practice every now and then, shouldn't my practice come first? Lastly, we come to a variable that carries much weight: proximity to performance. If it's just a soccer practice, but the soccer game is the next day, soccer practice wins. I ought to publish these rules. I think it's fair. I'd love to hear what you think.