It is quite common for schools to show PG movies to students without getting permission from their parents beforehand. This can be a troubling issue for parents who turn to Gateways to Better Education for advice on how to deal with it.
A friendly visit with the teacher or school principal is always the best first step in any attempt to dissuade a school from showing questionable movies. If this doesn't work, there are legal issues you can point out to the administrator.
We recently contacted the Permissions Department of Buena Vista Films, a division of Disney. According to a spokesman for the department, the school would need a public performance license in order to show any Disney film for any reason other than directly related to the curriculum. For example, if students were learning about Lewis Carroll's books, the teacher could show Alice in Wonderland without obtaining a special license. It would not be permissible to show the video just for entertainment.
It is reasonable to assume that other studios have similar restrictions and requirements.
Parents concerned about schools showing students entertainment videos as substitutes for academic work (whether G, PG, PG-13, or R) can alert the school principal to the illegality of the activity.