The practice is illegal. The movies you buy are for personal use only and not for showing to large groups of people, including students in classrooms. When you purchase the movie, you are not purchasing a license to show it to large groups of people. It doesn't matter if there is an admission or not. That being said, most people don't realize this and therefore Disney, as well as other movies, are shown in classrooms all over the country.
The United States Copyright Office states that, "If the movie is for entertainment purposes, you need to get a clearance or license for its performance." See:
http://www.copyright.gov/help/faq/faq-fairuse.html
Movie License USA also provides a list of criteria to determine if you need a license to show your movie at a school. From:
http://www.movlic.com/k12/faq.html#4
It's the third point that is the problem. Most times, Disney movies are being shown for entertainment value only, not as part of the curriculum.
Since it's pretty widespread, I'm pretty sure that TPTB at Disney are aware this is going on and have chosen not to do battle over non-admission showings of their movies in schools.