I haven't read all of the responses so apologies if this has been repeated.
I sit on 10 school governing bodies as a Clerk, the schools vary from privileged areas to deprived areas, academies to LA-maintained. All are in a pretty similar state.
Reasons are:
In the last year, the Unions agreed a 2% pay rise for teaching staff. This is on top of the cost of living award. I'm pleased they did agree it after such a long pay freeze for teachers, however, this has resulted in tens of thousands for each school which wasn't budgeted for. This has impacted on pension costs the schools pay too.
The number of parents applying for Free School Meals has dropped because kids get FSM up until age 7 now, meaning the school misses out on a great deal of funding (£900 per pupil) for those who don't apply.
The LAs are being cut left right and centre. Meaning, service the LA previously provided (training support, service level agreements for things such as cleaners, supply teacher services) are being cut, and schools are having to go externally to fund these, often at a greater cost.
Staff sickness and stress is on the increase due to immense pressure from Ofsted, therefore more money being spent on supply cover or longer term temporary contracts to cover posts.
There are falling school numbers. A few years ago, there were not enough school places to match birth rates. They increased school sizes, built new schools. Now there's a drop. Schools are still having to pay for full staffing, for larger buildings etc., and are now having to make redundancies and merge classes together.
There are loads more examples I could delve in to. Just last week I sat on a governing body meeting of a school who is not reaching targets but has to cut its staffing because of falling school numbers. This is a school who has already made cuts where possible, and has nothing additional budgeted for for next year's budget compared to this year's, but has a projected 6 figure deficit for the financial year due to the hike in costs of what they must pay, and the drop in pupil funding due to falling numbers.
I really feel for schools at the moment. There's so much to pay and no money to pay for it with.