A PG rating means the ratings board suggests parents watch the movie *first* and decide whether it's suitable for their children. It doesn't mean it's OK for kids as long as their parents are with them.
Our school is K - 6th grade as well but our school district will not let us show PG movies. It is extremely difficult to find movies rated G that will appeal to such a wide age range. We require the parents to attend the movie night with their kids.
We show PG movies. We put the movie rating on the flyer so parents can choose whether or not they will attend. Our school is K-6 so it is hard to find a movie that will appeal to all ages. So far we have not received any complaints about showing PG movies. We will next be showing Dolphin Tales.