Councils "top slice" budgets for maintained schools to fund the service they provide. The top slice happens before the schools receive their budget. This funding would be used for some council SEND provision - such as EPs or Behaviour support, but can't be used to fund statutory provisions such as Special school places.
Councils can not top slice Academy schools, so often Academies buy back into some council provision or provide their own for non-statutory services.
To fund statutory provision, such as Special Needs School places, the council has High Needs funding block as part of its Dedicated Schools Grant. This is taken from all schools (including Academies) before they receive their budgets. Usually the % or amount of funding is "agreed" between School's forum and the council with some guidance from the DfE in regard to how much is reasonable. However, increasingly, councils are finding that they have big deficits in the High Needs funding block and are seeking to increase the amount they take from schools.
This isn't a new thing- three years ago in my LA we were told that we had to fund an extra £15,000 per school as a one off or have a significant increase to the % contribution going forward as there was a £1 million in year deficit.
However, whilst not a new thing, the demands on the HNB are rising much faster than the budget available. The DfE has oversight of each council's HNB budget and will intervene (but not give them more money) if they are prediciting large deficits. The intervention is referrred to as a Safety Valve Agreement
https://www.lgcplus.com/services/children/dfe-agrees-20-safety-valve-deals-to-address-send-deficits-21-03-2023/