If the school policy allows only G movies (I know, there aren't many), then you obviously can't show it. If the policy is to allow PG but no cussing or nudity, then you obviously still can't show it. Is Ice Age out yet?
It is hard to find PG movies. A couple of years ago we all (all 600 of our 1st & 2nd graders) got to go see Horton Hears a Who at our local theater. This year Cloudy With a Chance of Meatballs came out and we were excited. Our principal said she would let us use one of our field trips for it. We waited and waited for them to rate it, and it turned out to be PG. Grrr... We couldn't go see it then.