Since this is a new school for your family, you are also trying to figure out what the school's culture and atmosphere are. You were expecting it to be more formal because it is a private school. You may be finding that the culture is very informal. You can check with your child's teacher and other parents who've been at the school longer to find out if the principal's informality is just his or if it transfers to the school's culture. Then you have a better understanding of what's going on and how to address it (or even if you should).
For the volunteer thing, I think you can talk with him to explain your sacrifice and how you would prefer to be notified in the future. Let him know how you want to be treated. GL!