If this invoice is old and out of your control, the company may be willing to cancel it or discount it to keep your business, depending on the situation. An invoice in the spring coming out of the blue means somebody screwed up. If it was your predecessor, see if your principal/school board/FA director maybe can help out. If it was the company, I would negotiate. If the screwup was yours, own up to it.
In any case, the company can and will wait a couple of months for next year's budget if that's what it takes. They would rather be paid late than never. The important thing is to communicate and let them know what's going on, otherwise their collections department will start calling your finance office. If that happens in my district it's big trouble.