you know one arrest there may send a message...and I am not one for arresting, but imagine alcohol at an elementary school, what would the ticket look like, and would YOU really want that on your record?
Short of hiring a permanent "litter nazi" for the school grounds, there's not much that can be done about litter without attracting negative attention that might lead to the course being pulled. I know I don't hold the most favorable opinion when it comes to the boys in blue, but I really don't see what calling the cops would do here unless you caught the person in the act and held them down until the police arrived. If you catch someone in the act of tossing a can on the ground, call them out on it. If they don't pick it up, THEN call the cops if you must, just don't be surprised if the cops take 30 minutes minimum to respond to your beer can emergency (perp probably gone by then anyway) because they're too busy fulfilling their proper roles as armed revenue collecting agents for Der State, ya know, keeping people safe by handing out tickets to drivers who haven't hurt anybody.
Some people just suck, man. Thankfully there are also those who don't suck and can pick up after those who do.