Our school district's policy (remember we're in the 5th largest school district in the US) is that we can only show "G" rated movies. It's not even an option. We're a preK-5 school. You might want to check with district policy as well.
I can see where you're coming from. If the 4th and 5th graders were reading Narnia for school it would make sense to see about showing the movie. The film ratings guide states that it is rated PG for battle sequences and for frightening moments. Perhaps if you are really interested in it you could ask the principal if you could send a letter home to parents stating that you are "considering" showing the movie as a tie in with the book that they have read. and ask for the parents input. Also, if they are aware ahead of time they could be suggested to preview the movie if they have not seen it already. Also, if they don't want their child to watch it then they simply would not bring them to the event. Our 5th and 6th graders went as a class to see it at the movie theater as a tie in to reading the book for class! It really prompted them to read more and want to read the rest of the series!!
We serve Kindergarten through 5th Grade. We talked about doing two different nights - one for the younger with G movies and one for the older with PG. Our 4th and 5th grader read Narnia and it would be an awesome complement to their reading but it's rated PG.
Personally, and I'm no prude, I just don't see the upside to picking that battle. There are so many choices (oldies, classics, current) that would allow you to have agreat night and still stay within the principal's concerns. I'm sure there are some parents in your school (even if you don't know them or if they haven't spoken up) who wouldn't like a PG event. Important to keep them in mind, even if the more well-known crowd has no problem with the PG.
What grades does your school serve? What are the grades targeted for this event?
We are wanting to show a PG movie during our Family Movie Night and are getting resistance from our Principal. Is there anyone in Williamson County, TN that has dealt with this? Anyone else have any ideas how we can make this work?