The way parents can stop this is by asking the school whether they had a license from Disney to show the movie.
The permission slips are just proof they showed the movie to a large group of people (most likely without a license, because what school has a budget for that?)
As with most legal matters, it depends: https://www.copyright.gov/fair-use/index.html
I don't know many schools willing to bother finding out whether the use was fair.