Schools and districts are funded from a lot of places. Some funding is local, some comes from the state, and some from the district. The most important part of designing a budget for a school is to look at what you need and then decide which funding source that should come from. For example, you can get computers from a technology fund or use some of the funding tied to ELLs because they are the ones who will be using the computers.
School budget is a very complex matter and varies from one state to the other. There are many things to take into consideration when doing a school budget. Generally your biggest expense is going to be Certified Salaries and Benefits. These are two areas that you really have very little control over. In most cases your insurance rates are controlled by the insurance companies and in some cases salaries are controlled by the state or local teacher organizations. The other frustrating part is you are never sure exactly what your Revenue is going to be, the state tells you you can expect this much, but then nit always is less than what they tell you at budget time. There is software out there that can assist with school budgeting, which makes somewhat easier to play with numbers. I have included a link to my states accounting manual.