Except, often, students plan without any sort of heads up from administrators/teachers/adults -- and I applaud them! As I've already said, schools are in a tough spot: perhaps personally supporting them, yet needing to uphold the code of discipline.
Each school has their own rules governing these things. There is no one answer. Wards of the state? We're responsible for them, but if they walk out, they walk out.
I have no idea what my school would do, to be honest. The immediate reaction would be: its a school/class cut.....that usually gets an office detention or Saturday school. They would likely not be allowed to make up any work they missed that particular day. (