Education is being "negatively affected" as schools struggle with budget shortfalls, a survey finds.
Morale is low and stress levels high, teachers, support staff and parents in West Sussex told the Unison union.
The situation could worsen next year as staff cuts are planned at 56 out of 330 schools in the next academic year.
The Department for Education said school funding was at record levels, but it was aware many schools still faced financial pressures.
Cut back heating
Unison said the survey showed "staffing restructures leave the most vulnerable children with reduced provision" and were "often poorly managed".
The union said schools were also facing cuts to equipment and school building maintenance.
In a statement Unison said: "It was also concerning that some schools were cutting back on heating to make ends meet. We would be very concerned that some schools were, in effect, suffering from fuel poverty."
The union said there was "simply not enough money to educate children".
Unison spokesman Daniel Sartin called for "enough support staff to support children in school classrooms, particularly the vulnerable children who may have disabilities or behavioural issues".
Schools in West Sussex have been at the forefront of a national campaign for a new national funding formula - with head teachers warning that they face cutting teaching staff, merging classes or reducing school hours.
Unison said its survey of more than 500 people confirmed there was "a direct link between cutting jobs, overworking our members and dwindling resources and a corresponding negative impact on children's educational outcomes".
About half of the respondents said pupils' attainment had got worse, while 65% said special educational needs provision had suffered as a result of restructure in their school.
A teaching assistant specialising in children's welfare and special educational needs told the BBC her school in West Sussex was planning to end her role.
Speaking anonymously, she said: "The children in my school will not have access to somebody that they can talk to. They will continue to be abused. They will continue to cause disruption in the class. They will continue to stop other children having a right to their education.
"So every single child will be affected because these children don't have somebody to turn to and don't have someone that can just spend that time listening to them."
A Department for Education spokesman said school funding was at record levels, but it "recognised schools are facing cost pressures".
"We will continue to provide support to help them use their funding in cost effective ways," he added.
West Sussex County Council said it was aware of the financial pressures and would continue to work closely with schools "to offer support and help them through this difficult time".