We've usually tested our kids out and gave them the top three they were successful at. Sometimes the parents like the choice to get that feeling of having a say.

For percussion, we used to do tests and if parents argued we would show how they did on the rhythm tests and what the minimum score needs to be in order to be percussion.

Of course, it can all be for naught if your admin won't back you up. Whatever you do make sure they're with you.