Another idea would be to call your district office. They may have larger screens or equipment that can be checked out to the school. I was looking into showing a movie that hasn't gotten to vhs/dvd yet and was told that I could only get it on 16mm. I know we don't have the equipment at school for that, but the county office tech does.
You could also use white bulletin board paper (which you should have on campus) for a screen. Our grade level put some up between the posts in our hall and had a movie night outside for our first grade campout. The kids loved it.
It's called an LCD projector. Your school library probably has one (or more) of these. Most classes use them to hook up to a laptop to show powerpoint presentations...but a dvd player can also be hooked up to the lcd projector to show movies. We just use the pull down screen in the cafeteria and the plain white wall in the gym.
Do a search on these message boards for "movie night" - you'll get lots of tips from other parent groups that have had great successes from their movie nights and tips on things to avoid.
Please help...I absolutely love the idea of Movie Night, but I just haven't been able to find anyone to help out in regards to a projection device. What are all you guys out there using? Are you parents donating something, or are you renting something? Please let me know...I know this would be a great activity and I'd really like to know how to go about getting this off the ground.