Funding cuts have forced up class sizes in secondary schools across England, according to leading education unions, as schools balance their budgets by cutting staff.

Using official figures published by the Department for Education, the coalition of unions found that 62% of state secondary schools had larger classes last year than two years before.

“We have repeatedly warned that schools have had to increase class sizes because of funding pressures and here is more evidence that this is the case,” said Geoff Barton, general secretary of the Association of School and College Leaders, one of the unions involved.

“It is the last thing schools want to do but they have no other choice because they have to reduce staffing numbers and that inevitably affects the teacher-to-pupil ratio.”

Barton, whose union largely represents secondary school heads, said that larger class sizes meant less individual support for students, and more pressure on teachers.

Among the worst affected were schools in York, where the average class size rose by nearly three students per class throughout the local authority, rising from 18 pupils in 2015-16, to more than 21 last year.

But the Department for Education (DfE) strongly disputed the group’s conclusions, calling the use of figures “fundamentally misleading”.

“We are investing an additional £1.3bn in schools funding over and above previous plans, so that spending on schools will rise from just under £41bn now to £43.5bn in 2019-20,” a DfE spokesperson said.

“As the independent Institute for Fiscal Studies has said, overall funding per pupil across the country will now be maintained in real terms up to 2020. We have also protected the base rate of funding for all 16-19 year old students until 2020.

“We have invested £5.8bn in school buildings, creating 735,000 places since 2010 and despite rising pupil numbers, the average class size has seen little change.”

The figures used by the unions’ School Cuts campaign suggest that although the number of pupils in more than 2,700 schools was little changed, the income received for each pupil fell in real terms from £5,396 to £5,173 in the space of three years because of inflation, rising wages, taxes and pension contributions.

Nationally, the average class size last year was 20.8 pupils, compared with 20.1 in 2015.

Schools that already had the largest class sizes – in Barnsley, Thurrock, Newham and Leicester – saw further increases, with Barnsley packing more than 24 pupils in its classrooms, and Thurrock more than 23.

“As class sizes increase and targeted professional support by teaching assistants and other support staff is cut, once again it is the pupils who lose out,” said the GMB’s Karen Leonard.

“If we don’t stop these cuts, we run the risk of lessons reverting to a Victorian ‘one size fits all’ model where any child with additional needs, gifts or talents, or just needing a bit of extra support, simply won’t get it.”

Just 21 out of 150 local authorities saw a fall in class sizes. The largest reduction was in Salford, where average classes fell to 20.5 pupils.

The National Education Union – the merged National Union of Teachers and the Association of Teachers and Lecturers – has marked International Women’s Day by calling for female teachers to receive the same pay as their male counterparts.

The average pay for female teachers is £2,900 less than for men, while women in leadership positions earn £5,700 less than their male counterparts, according to department figures.

