Pixie, I do sympathise with the issues around teachers needing a break, and managing large groups of children continuously is a feature of our state schooling system. Whether this is good, bad, desirable or whatever is another discussion.
But putting in a commercial DVD is not the solution. You're saying that there's no harm in it, and I disagree. I'm concerned that as a teacher you seem to be unaware of the raging debate regarding the appropriateness of film and tv for children.
Personally, I don't believe Hollywood has my children's interests in mind, I think they want to make as much money as possible and if they can get a lovely well-respected actress and a charming story line that will appeal to parents, all the better for soothing concerns. Must sell merchandise. This crap does not belong in schools, period.
We've visited two local schools so far, and one of them says yes, they use films, sometimes as 'educational', but that we could talk to the teacher and she'd let us know to come collect dc at those times. The second school was horrified by the idea of showing films to children, and said "we don't do that here." (Guess where we're sending ours!)
Also, the whole idea that the child should be free to remove themselves from the situation if they desire is great in theory, but what child will actually do this in a class situation? How often are they empowered to make these sorts of decisions for themselves in other school situations, and are they even aware enough to be able to do so? Not a four year old, who will instead trust the adult to take care of them.