Invite all the neighborhood kids over for a goodbye to summer bash by planning a backyard movie night. Bedtime comes sooner once schools starts, celebrate the beginning of school by staying up late one last time.
Have the kids make flyers and deliver them to their friend's houses in the neighborhood.
Use a white sheet and attach it to a swing set, wall, deck or fence using clamps and rope.
Maybe you own a projector? If not consider asking to borrow one from your place of work or church. You could also rent one.
Choose a movie with a "School" theme. Some suggestions are,
Back to School with Franklin (G)
Akeela and the Bee (PG)
School of Rock (PG 13)