The school funding crisis is being felt all over the country, with cutbacks, redundancies and parents being asked to donate cash, pencils and glue sticks. But at one school in south London, teachers have been on the picket lines throughout the year. Is this a taste of things to come?

It is 8.30 on a Thursday morning. Ordinarily, this leafy street in south London would be teeming with life, as boys clad in black blazers and striped ties make their way to school. Today, the road is uncharacteristically quiet.

It is the 13th day of strike action since November by teachers at Forest Hill School for Boys in Lewisham, and most of the 1,400 pupils are at home again. Their teachers are protesting against staff cuts designed to save the school £1.3m. Fifteen teaching jobs are being cut from September, more than 20 support staff have already gone, and the head teacher is now grappling with a rash of resignations from disillusioned teachers who want out.

A small group of striking teachers gather at the school gate. A few boys amble into the building for year 10 exams, music practice or a trip to Whitstable, as a van arrives with placards and a union banner is unfurled. The mood is subdued but picks up when someone finds the megaphone. The chants begin and energy levels lift: “No ifs, no buts, no Forest Hill cuts”; “They say cutback, we say fight back”; and “Not our deficit, it’s our school, don’t mess with it.”

Earlier in the dispute, the protests to highlight the plight of the school were bigger, but as the end of term approaches, numbers have diminished. Today, the gathering of staff is bolstered by a few parents who turn out in support – some with children – and there are regular toots from passing vehicles.

“It’s a fantastic school,” says Jacqueline Morrish, who has a son in year 9 and has turned out this morning to support his teachers. “It’s been brilliant. They treat the boys really well, and they flourish.” One father, whose 15-year-old son is one of the few attending school today, is also fully behind the striking teachers: “My boy is on the autistic spectrum. They are very good with special needs. That’s why he’s here. It’s been excellent.”

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Phil Beadle, a former teacher who now writes books and works as an education consultant, has two sons at the school. “The teachers have the boys’ best interests at heart, have always conducted themselves as models of integrity and are striking to protect the education of the young people in their care. They would not have taken the decision to strike lightly.”

Elsewhere, however, patience among some parents is running thin. “My son’s year 8 has been trashed,” says one mother, who does not want to provide her name. “I was supportive of the ideals of the strike initially, but I’m not any more. I see the damage it has done to my son.”

The dispute in Dacres Road SE23 has so far attracted little national attention, but in a climate of dwindling school budgets and education cutbacks, the story of Forest Hill will resonate with teachers and parents up and down the country. Its plight has been described by some as the canary in the education system – a warning of what is to come as more and more schools struggle to balance their budgets and fall into deficit.

The same theme is already beginning to play out across the country, with innumerable stories of schools cutting back on everything from teaching hours to photocopying, and even begging for money from parents. Danemill primary school in Enderby, near Leicester, is just one of a number of schools proposing a four-and-a-half-day week to cut costs. Elsewhere, parents of pupils at Davenant Foundation school in Loughton, Essex, were asked for £20 donations to cover the costs of “creative studies” – including salt and milk for food technology lessons. At St Nicolas’ Church of England school in Taplow, Bucks, parents received letters asking for contributions of up to £30 a month to plug a projected £40,000 shortfall next year or face staff redundancies. Meanwhile, St John’s primary school in Crowborough, East Sussex, is not alone in asking for the most basic contributions from parents – including toilet rolls, glue sticks and pencils. Children at Furzedown primary school, in the London borough of Wandsworth, are vacuuming their own classrooms because the school can’t afford to replace the cleaner.

Teaching unions say the crisis is the result of years of frozen budgets, further eroded by higher pension, wage and tax costs, as well as inflation. As a result, the National Audit Office (NAO) calculates that schools are facing a £3bn real-terms cut in funding by 2019, with more schools going into deficit as a result. NAO figures show that the proportion of local authority-maintained secondary schools spending more than their income increased from 34% in 2010/11 to 59% in 2014-15, while the average size of deficit increased from £246,000 to £326,000 during the same period.

The growing schools funding crisis became a key issue in the general election campaign, forcing the Tory government to listen to parents’ and teachers’ concerns. On Monday, the education secretary Justine Greening announced an additional £1.3bn in schools’ funding to try to ease the pressure on budgets and placate angry parents. Teachers’ leaders say it is nowhere near enough. Given the scale of the deficit at Forest Hill, it is unlikely to have any immediate impact on the current crisis facing the school. Meanwhile, uncertainty remains over plans to reform the way in which schools are funded nationally, which could still have an additional impact on London schools.

Founded in 1956, Forest Hill school has been a popular, successful and highly regarded boys’ comprehensive that has served its diverse local community well. The original London county council brick structure has been replaced in the last decade with a modern PFI-funded building, which is shrouded in a distinctive palette of yellow and gold panels and cost £23.5m. The school’s reputation has been one of high achievement and inclusivity, catering for students of all backgrounds and abilities; its record on pastoral care and special education needs is second to none.

Facebook Twitter Pinterest A march to defend schools in Bristol in May. Photograph: Chapman/LNP/Rex/Shutterstock

Zoe Camenzuli looked at 20 different schools for her son Joshua, 12, before deciding on Forest Hill, where he began last September. “I walked in and I instantly felt comfortable,” she says. “The boys are really well behaved. The teachers and pupils have such a good relationship. There was a really special feel about the school. I got the feeling they really cared about the boys.”

“It’s everything you want from an inner-London comprehensive,” says Martin Powell-Davis, the National Union of Teachers representative for London, whose own children attended the school. “It’s just what parents are after. Good support for the more academic pupils, but also a good social mix. It’s a genuine comprehensive with a stable staff working because they care for the kids.”

Forest Hill has often been over-subscribed. Until recently, it seemed a picture of good health – a “good” school, according to the schools watchdog Ofsted [pdf] , with an experienced team of committed staff who took up jobs intending to stay a few years then move on, but ended up building their entire careers there.

Then last year, unexpectedly, Forest Hill saw a sudden drop in its GCSE results – the percentage of pupils getting a C or above in maths fell from 71% in 2015 to 51% in 2016, while English went down from 68% to 54% – partly because of a lower-ability intake. In November, a Lewisham council document revealed that at the end of the 2015/16 financial year the school had overspent its budget by £129,000 and was projected to be in deficit by £879,000 by the end of the 2016/17 financial year. The council was asked to approve a licensed loan as part of a financial recovery plan that required a staggering £1.3m cut in staff costs.

Parents remember getting an email from the school referring to a restructuring, but they remained largely unaware of the severity of the situation until March, when the scale of the deficit and the action required to get the budget back in balance was revealed at an open meeting. Phil Beadle, who attended the meeting, blogged – in some shock – afterwards: “I’ve just returned from an open meeting at which a packed audience of parents applauded their children’s teachers for voting in favour of strike action.”

The NUT, whose members are striking at Forest Hill, says 23 teachers have now handed in their resignation ahead of the new term in September; there have been five teacher redundancies and three temporary contracts have not being renewed. “Together with support staff, that means over 50 staff will have left this year,” says Powell-Davies.

Remaining teachers are being asked to teach more hours – up from 19 hours to 22 – allowing less time for planning, preparation and assessment. Sample timetables for next term have shown some staff teaching solidly for four days; the curriculum is being “decimated”, in the words of one parent, and special needs and pastoral care – once regarded as a particular strength of the school – are being eroded.

Earlier this month, local Labour MP Ellie Reeves, in a letter to Justine Greening, called for more funding, warning that the high standards and strong community at the school were at risk. Meanwhile, the school’s punishing PFI payments, which eat up 10% of the entire annual budget, continue to drain funds. Such are the constraints of the contract that the school’s use of the building is limited to agreed hours – to use it at other times would cost £200 an hour, hampering fund-raising efforts.

Headteacher Mike Sullivan declined to give an interview but issued a statement: “Forest Hill school’s large deficit has made it necessary to reorganise all aspects of our staffing in order that we can live within our means in an era of decreasing government funding and increasing costs.

“The chair of the governing body and I are new in post and we are determined to put our school back on a firm financial footing and we are doing this without risking the education of our students. In my conversations with parents this year, the overwhelming majority have expressed support for the school and its future.”

History teacher Joe Cowley is leaving Forest Hill after six years – not just because of the restructuring – and he says he is horrified at what is happening to the school, and the funding of education nationally. “This is a fantastic school – it’s a real community education. But the restructuring here is a microcosm of what’s going on more broadly in education. It’s incredible that a school of this calibre is being put under such strain, because we can’t, as a country, afford to properly fund schools and working-class education. This isn’t happening at Dulwich College or Alleyn’s [both independent schools in the area].”

Forest Hill’s current predicament is partly a consequence of the funding constraints affecting all schools. It was particularly badly affected by cuts to post-16 funding, losing £150,000 from its annual budget. But the council claims that other specific factors have played a part – one document suggests that teaching costs at Forest Hill are £125 per pupil above the average for similar-sized schools within Lewisham.

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There have also been claims that, historically, the school’s financial management has “been unable to adequately monitor the school’s budget position to allow early identification of issues”.

Nevertheless, Forest Hill is not the only state school in Lewisham to be in financial difficulty. Prendergast Ladywell school, a mixed secondary in the south London borough, overspent its budget by £173,000, according to a council document dating back to November. The same document reveals that eight secondary schools were in deficit at that point, up from two in the previous financial year.

As the summer holiday approaches, many Forest Hill parents are worried what the new academic year will bring and who will return. “People are already leaving,” says Jacqueline Morrish. “Someone we know has just taken both their boys out and is taking them off to private school.” Pupils are also upset. “My son is gutted because all his teachers are leaving,” says Kester Brewin, whose son is just finishing year 8.

“It’s kind of annoying,” says Camenzuli’s son Joshua, who is at the end of year 7 and, instead of being in lessons, is on the picket line with his mother. “I picked this school because I thought it would be a really good school to learn. I liked it because it’s a performing arts school. I like most of the teachers – they’re really nice. Now I’m worried it will affect my grades.”

Another 12-year-old, who is leaving the school because his mother is concerned Forest Hill can no longer meet his special needs, says: “It was really good when I started. I felt really comfortable then, and now I don’t.”

He has a range of special needs because he is autistic, has ADHD and Tourette’s syndrome, and is finding the changes difficult to deal with. “There’s lots of change. There’s no more Room 21 [a room set aside for children in need of time out of the classroom]. I’m not staying at the school. It’s not helping.” As yet his mum has not found him anywhere else to go in September.

Facebook Twitter Pinterest Parent Zoe Camenzuli with son Joshua: ‘It’s not the responsibility of our boys to pay the price for bad decisions made by other people.’ Photograph: Graeme Robertson for the Guardian

Some parents blame the council for failing to keep a closer eye on the school’s budget. But councils, too, have been subject to severe funding cuts, inevitably affecting their capacity to scrutinise school spending. “It’s a perfect example of the kind of endgame of austerity,” says Brewin.

Lewisham council also declined to be interviewed, but provided this statement: “We are doing everything we can to support the headteacher and chair of governors of Forest Hill school to raise standards and bring the budget back on to a secure financial footing. Talks are progressing between Forest Hill school and the unions and we hope these talks will result in an agreement that avoids further strike action.”

“I’m still in shock,” says Camenzuli, sounding tearful. “I feel powerless. Our boys are not responsible for bad decisions made by other people. They have one shot at their education. Why should our sons pay the price?”