Find out what is up in the sky. The moon and planets guarantee a great viewing night, even if a light cloud cover is present. Light pollution from most school yard lights won't bother views of the moon and planets too much. Don't expect to see many deep sky objects from a schoolyard, however. Double stars are great targets from bright school sites, and so are a few large clusters or nebulae.
Most galaxies are too small and dim for young eyes and bright lights.
Plan your events during the time of the year when daylight savings is not in effect. The most successful star parties occur when it is dark early in the evening. Allow the astronomers to drive in to the blacktop area to unload their equipment if at all possible. They will not be very enthusiastic if they have to cart delicate equipment a hundred yards and back. They will swoon with pleasure if they are allowed to leave their cars nearby their telescopes.