Schools in England have warned they expect to get less money next year than ministers have promised.
The government said every secondary school would receive at least £4,800 per pupil in 2019.
But councils have transferred money from school funds to boost budgets for children with high needs.
A Department for Education spokesman said there was more money for every pupil in every school.
Dave Baker, chief executive of the Olympus Academy Trust - which runs nine schools outside Bristol - said parents felt furious and betrayed because promises had not been met.