Luckily, this is researched extensively, and schools must publish budgets. Typically, spending on 'instruction' is >60% of the expenditures, almost all of that is salaries and benefits. Capital outlay is usually around 8-10%, support services and administration around 10%, operations and maintenance around 10%, and around 5% for transportation/bussing.

Contrary to the typical claims that 'administration' or capital costs suck up all the money, the majority of the money does go to teachers' salary.