If you take any school budget, and strip away everything that is not an actual classroom teacher, you will find that ~1/3 or less goes to "frontline" teaching costs.
Another 1/3 goes to special ed and all that is attendant with that. I mean, my district BOUGHT a car and hired a FULLTIME driver for one student who had to be taken to special programs. You have 1:1 class aids for many kids. Special ed is < 10% of kids, and even then the huge costs add up for the 1%. This is a massively subscale operation where every school is legally obligated to deliver services.
Then you have the last 1/3 which is everything else. Food, facilities, sports, admin, transportation, etc. Admin is actually a leaner slice than most unless you are getting into really small schools where you have a principal on top of the teachers and that adds significant salary. In bigger schools this fades away with scale.