While schools across the country are pressed for funds nothing justifies this sort of behavior. Even if it were legal, and it wasn't according to Board policy, it treads a mighty thin ethical line.
In Michigan the state Supreme Court has ruled that things occurring during the school day are part of the educational experience and since the Michigan Constitution says public education is "free" then there can be no charge. That includes field trips that occur during the school day.
I'd be interested in what you think about this principal's action.