It all starts with alumni. Then businesses in the area. Get the school to send out email broadcasts and you all get the emails from the chamber for businesses. This saves stationery, postage, etc. Chamber might even be willing to sponsor an event since the school brings in revenue for the county with salaries, etc.
Have an event at the field so people can see how bad it is. Doesn't have to be much, maybe a "covered dish" dinner with the school or others providing meats and guests providing the "fixins". Or a grilling cookoff. Charge for plates of food. Get a local band(lots of bands will do for free for the exposure) or the school or HS band to be on hand. Or maybe a DJ. Music puts all in a good mood. Local newspaper can advertise usually for free or AT LEAST do a PSA, same for radio. Get businesses to donate items (or sponsor an event) and have a live auction. Or silent. (you make more on live)
Or organize a golf tourney. Who provides uniforms, softballs, etc. Maybe they would sponsor an event. Anybody that the school buys stuff from should contribute or maybe sponsor an event. Not knowing exactly where you're talking about, just hard to say. If there's fishing around, maybe a fishing tournament.