What is the best way to attract the most birds in a small amount of time?

I am a senior in a high school where it is common for senior pranks to happen. My senior prank involves having a bunch of birds be in the main lawn area of the school. The problem is is that I need a bunch of birds to show up, but they need to show up in a short amount of time for this to work. I would prefer the solution to be cheap but if not cheap at least affordable for a high school student making minimum wage.