1. Your chosen school probably already has copies of those books (although they won't say no to more / less battered copies).
2. Probably some of the children have already read them.
3. Certainly, when the film of The Dark is Rising comes out, it will increase the number of children who read the book, and will promote discussion of the book amongst the children and within the class.
4. Whether or not it's true, many of the children will also express the opinion that the book is better than the film. This will help them feel good about reading, and encourage them to read even more.
School children read and enjoy good books as much as they ever used to. Many teachers also read and enjoy children's books, and recommend good examples to their class. Some of those examples are recent, and some are older. School libraries keep copies of good children's books, including older books, and display them in prominent positions. The children in my school have certainly had the opportunity to read The Dark is Rising, and some of them have taken it.
Nonetheless, I am sure that more children will read the book in the run-up to and aftermath of the film. When Narnia came out, there wasn't a child in my class who didn't read it or have it read to them. We had a great discussion about differences and similarities between the film and the book (something which is very much built in to the current English curriculum, and rightly so, because it makes people think very deeply about aspects of the story). More recently, five or six children from my class have been reading Harry Potter and the Order of the Phoenix, some of them in preparation for the film, and some of them as a response to seeing it. Some prefer the film and some prefer the book, but all are happy to discuss their reasons in detail.
So, I have mixed feelings about the upcoming film of The Dark is Rising. Personally, I may be very disappointed by its treatment of my favourite-ever children’s book. From the point of view of a teacher, if it is a well-made children’s film then I will welcome the fact that it raises the profile of the book and provides interesting opportunities for discussion and comparison.