I would check with the librarian, as FoxMom says. A sheet will work; you might be able to borrow a projection screen from the school district. We have a laptop with a DVD drive, and we hook it up to an LCD projector. Our school has the projector; lots of companies have them for meetings, you might be able to borrow one.
You may or may not be able to hook the laptop directly into the gym speakers; one year, someone held the microphone next to the laptop's little speakers. This year, I did buy a 50' cable that had a standard headphone plug on one end and RCA plugs on the other, because our sound system has RCA inputs. Microphone inputs typically have extra amplification, so you may not be able to plug in there, and if you do, watch the volume.
A license is $75 from
Public Performance License for Movies | Servicing Schools & Libraries Worldwide As I understand it, you may charge admission to cover the cost of the license. We did a canned food drive instead.