A school will be forced to axe several teaching assistants as it does not have the money to keep them on.
Stephen McCulley, head of St Peter's CE Primary, in Henfield, said he felt "desperately unhappy" about the move, which was announced to parents and staff yesterday (February 28).
St Peter's receives less than £3,000 per pupil in government funding each year - well below the national average - and currently has 19 teaching assistants (TAs) on its team.
A consultation has been launched into the proposal to make as many as six of them redundant, which Mr McCulley said he hoped would be completed before Easter.
He said: "I feel desperately unhappy.
"Obviously the human side of anyone losing their jobs is regrettable and unfortunate, but the impact on the development of the school and the children and their education is disheartening."
Mr McCulley said there had been "upset and anger" when he announced the news, but that no staff would be made redundant before the end of the school year.
He added: "We're not going to cut them loose but I wanted to get things done as expediently as possible so people have time to search for other jobs."
Describing the loss of so many valued colleagues as "immense", he pointed out that the remaining staff would have to take on the workload left behind by the TAs.
Describing that workload, Mr McCulley said: "They support the children in their learning, do one-to-one with children with special educational needs and those with English as a second language, and just generally everything that needs doing.
"That's apart from the day-to-day humdrum stuff of helping children out with their PE kits.
"The impact of the classrooms is going to be immense."
Mr McCulley joined St Peter's in September and quickly realised redundancies were a very real possibility as the school - like so many others in under-funded West Sussex - attempted to make its budget stretch.
He and the governors came to the decision there was "no other way out" when they met on February 2.
The situation at St Peter's is one that has been echoed in schools across West Sussex, which have been fighting for fairer funding for almost two years through the Worth Less? campaign.
The local authority is one of the lowest funded in the country and will remain so even after the government's new National Funding Formula comes into effect in 2018.
While West Sussex funding will rise by around £79 per pupil - in a county where schools are already £402 per pupil short of the national average - rising costs will quickly swamp that, leaving schools even worse off.
Heads have warned of cuts to staffing, the curriculum and even school hours as they attempt to make ends meet.
Mr McCulley said: "It's just unacceptable. The government claims that school funding is at its highest level but that's not true because of rising pension, national insurance, fuel costs and supplies.
"Schools have been facing an 8-10 per cent budget cut year-on-year.
"We've been propping this up. Our reserves have gone as low as we can take them."
He appealed to parents to contact Arundel & South Downs MP Nick Herbert and put pressure on schools minister Nick Gibb MP to recognise the need for more money for West Sussex schools.
Mr Herbert can be contacted at email@example.com or 020 7219 4080.
Mr Gibb can be contacted at firstname.lastname@example.org. The number for the Department for Education is 0370 000 2288.
A consultation into the National Funding Formula will end of March 22. To take part, log on to consult.education.gov.uk/funding-policy-unit/schools-national-funding-formula2 .
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