Ive just sent the 3rd of my 5 kids to college, and they had to have a permission slip as high school football players to go home with their parents rather than ride the school bus. They needed notes for picnics, notes to buy lunch at McDonalds on field trips, they seek permission for the simplest things. Why? because our minor children are in their custody at these times, and a permission slip is implied contract to not hold them responsible for a variety of things.

This coach had minors in his charge. It was incumbent upon him to seek permission before doing this, because he is responsible for them, if not as a school representative (which was implied), then as a private citizen he is taking responsibility for all those minors, and any and all liability that may ensue. 

"Life's rich demand creates supply in the hands of the power, the only vote that matters." --James Michael Stipe, from Begin the Begin.