Primary places amounting to the equivalent of hundreds of schools will be unfilled over the next three years if current population trends continue, a Tes investigation can reveal.

Analysis of Department for Education data shows that as a population “bulge” from the 2000s moves up into secondary schools, primary school enrolments will fall by around 100,000 pupils by 2022.

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The birth rate has fallen since 2016, with 657,076 babies born in England and Wales in 2018 – the lowest number since 2005. Some schools in metropolitan areas are already seeing their numbers fall.

Primary heads warn of serious repercussions at school level, including:

• Schools being financially stretched to "breaking point", with squeezed budgets – through less funding per pupil – exacerbated by the school funding crisis

• School closures

• Teacher and support staff redundancies

• Bigger class sizes as schools compress year groups of three or four forms into fewer classes to cut costs

• Increased competition for places between schools

• Less collaboration between schools

• Schools in urban areas and those with lower Ofsted ratings being impacted most severely

• An unwillingness to discuss the issue, because of fears that parental perceptions will only make the issue worse.

The news will raise further concerns after the NAHT headteachers' union published survey results suggesting that 73 per cent of the many heads of small primary schools fearing closure cited “low or fluctuating pupil numbers” as the reason.

Furthermore, heads are warning that a decrease in primary school pupil numbers will have a significant impact on their budgets, exacerbating the existing school funding crisis.

Paul Whiteman, NAHT general secretary, warned that a fall in numbers could quickly plunge schools into crisis.

“We know that some of our primary school members are beginning to feel the effects of the bulge in pupil population having moved from primary to secondary," he told Tes.

"They are expecting fewer pupils in coming years but, particularly in smaller or less popular schools, this drop in numbers can be very sudden.

“It means schools are unable to accurately predict their budgets for the coming year, and cannot plan accordingly. Set against a backdrop of the school funding crisis, this means schools can go from just about managing to breaking point almost overnight.”

Tiffanie Harris, primary specialist for the Association of School and College Leaders (ASCL), said: “There probably will always be demographic fluctuations. However, as the level of funding is low in primary, any fluctuation, even a minor one, has a major effect. Although funding has been addressed, it’s still a long way off.”

A fall in pupil numbers will also lead to larger class sizes, as cash-strapped schools balance their books through staff redundancies.

According to Anne Lyons, former president of the NAHT, many schools are already reducing entry levels by one form following expansion because of the population “bulge”. This can lead to schools cramming a year group of three smaller classes into two, or even mixed-age classes.

She said: “I have a number of colleagues who expanded their schools by a form of entry in the last five years and now are mothballing that extra year group.”

“It might be they had three classes of 30 each, so 90 children, but they go down to 70 children. Now that is more than two classes, but not enough for three, and that funding wouldn’t be enough for three teachers,” Ms Lyons said.

“In many schools they are restructuring their classes – they might even have mixed-age classes of Year 2 and Year 3. It’s very difficult to sell to parents, because even though schools have good procedures, parents worry that if they have a Year 3 pupil in a mixed Year 2 class they won’t be working at the appropriate level.”

The spare capacity is already leading to staff redundancies. Gerald Clark, secretary for the NEU teaching union in Camden, North London, said that schools were already making teaching staff redundant.

“There are schools where teachers or teaching assistants have been made redundant. In some cases, when teaching assistants leave they are replaced with agency staff, whose contracts are then discontinued,” he said.

"Ultimately, a school has to have one teacher in front of 30 children. If that is your bare minimum calculation, I think some schools are thinking, 'Let’s put that in, and anything else is just a bonus'."

The issue will also affect collaboration between schools. Mr Clark said that “while heads in the borough want to work together, when you know you have a school down the road competing with you for places, it makes that more difficult”.

Timo Hannay, a statistician at education data website SchoolDash, analysed data from the DfE to predict the shortfall of 100,000 pupils. He also found that the issue disproportionately affects schools with lower Ofsted ratings, as well as those in urban areas where competition for places is more intense.

Comparing school occupancy rates in 2019 with those in 2015, he found that occupancy in “outstanding” schools had fallen by 1.9 percentage points, whereas in “requires improvement” schools it had fallen by 3.5 points – and for “inadequate” schools, it fell by 3.9 points.

“This isn’t affecting all schools proportionately – for some schools with lower Ofsted ratings, they will probably be forced to close,” he said.

In London, Mr Clark reported that the impact is already “massive” in Camden, “because a lot of our schools aren’t full, so their funding for a school drastically reduces, and that means the school has to look at how they can manage over the next three years with less funding”.

With the problem set to get worse over the next few years, primary schools will face drastic decisions regarding class sizes and staff retention in order to stay afloat.