Ellie Jones, 40, is an assistant secondary headteacher. Most mornings she gets up at 4 to do paperwork, arrives at school for 7.30 and gets home at the earliest around 6pm – often later – despite only teaching 11 hours (half a full timetable) a week. “I probably have around four or five hours sleep a night,” she says. At weekends she tries to have a full day off. She rarely manages it.

Jones, who has been teaching for 17 years, recently resigned her £52,000-a-year post with no job to go to. “I love the kids and teaching but I cannot maintain this for another 20 years. I’d break. They’d take me out of there in a box,” she says.

She is just one of the “outflux” of experienced teachers contributing to the recruitment crisis in schools. According to a recent National Education Union survey, 80% of classroom teachers have seriously considered leaving the profession in the past 12 months because of their workload. And a recent online poll by Teacher Tapp has found that only half of teachers reckon they’ll still be in the job in 10 years.

Emma Kell has written a book, How to Survive in Teaching.

Such losses are unsustainable for any profession, says Mary Bousted, joint general secretary of the National Education Union: there is a current shortfall of 30,000 in teacher numbers, and though this year the primary recruitment target was met, only 80% of the teachers needed for secondaries were enticed to join the profession. Bousted wants to see a statutory limit on teachers’ working hours, and a revision in what schools must measure.

The increase in workload is caused by government policy innovations, including significant changes to how pupil progress is assessed at both primary and secondary level – a programme that has been “far too hastily implemented”, says Bousted.

In the space of 48 hours last week, dozens of teachers who have recently resigned – or plan to before the 31 May deadline they must adhere to if they want to leave at the end of the school year – responded to a Guardian request to share their stories about why they were leaving.

Most cited workload as their number one reason. One primary teacher shared how she was forced to go into school during the holidays to supervise Sats revision. Another told how she sleeps for most of Saturday to recover from the hours she has put in during the week.

One primary teacher confessed to barely seeing her two-year-old son, rushing to get him to bed on weekday evenings so she could get on with marking and planning for the next day.

Over-testing – and pressure to meet government targets – was another common reason for leaving, with one former maths teacher saying she was forced to make a bottom set of year 7 pupils sit GCSE papers, supposedly so they could compare their marks at age 11 and 16. She found it “completely unethical”.

Pressure to focus on the progress of the highest achievers led to the resignation of one primary school teacher: “So we’re giving up on children at the age of four or five; so it’s ‘you go stay and play on the carpet there while we concentrate on the ones who might make the level’. I think that’s immoral.”

Some teachers shared harrowing stories of poor pupil behaviour and lack of support from senior management. “You do your best, but if you have three or four children in a classroom who struggle to be there, every lesson is a battleground,” said a teacher, who has recently left.

Another example is Sarah Tenter, a year 3 teacher with almost two decades’ experience who left her job at Christmas. Despite having three children in her class with autism or ADHD, and three more with “major issues”, she had only part-time support from a teaching assistant.

As her stress levels rocketed, she began to take time off school. “If one of my own children was poorly, instead of sending them to Grandma I’d say ‘Don’t worry, Mummy will stay off’. Which is awful because it had always been my dream to teach.”

She now has a part-time job working in her sister’s hotel business. Would she go back to teaching? “No,” she says.

Damian Hinds, in his first major speech since taking over as education minister in January, promised major reforms on workload – particularly around marking, planning and data management. But the general secretary of NASUWT, the National Association of Schoolmasters Union of Women Teachers, Chris Keates, says teachers need more. “It’s no good ministers like Damian Hinds wringing their hands and saying ‘I feel your pain’ over workload. What [teachers] want to see is tangible solutions and action taken.”

Headteachers must also take responsibility for reducing workload, says Keates. Ofsted announced last year that headteachers would routinely be asked how they reduce teacher workload. This followed the publication of a document [pdf] that stated teachers do not need to following a particular marking system or style of lesson planning – a mistaken belief that can be the source of stress in the profession.

“We distributed it to every single member and advised them that if there was a problem in their school to go to their headteacher. And some members have told us their head turned round and said ‘Ofsted may not require it, but I do’.”

But not every teacher who wants to resign can. Gemma Vine, a former geography teacher and single parent of two, had to stick it out until her children had left home before she resigned from her job – and a £40,000-a-year salary.

Teachers’ stress is compounded, Vine says, by knowing that “at any moment, if you’re not keeping up with your workload, you’re six weeks away from being dismissed on competence”. So as well as being overworked, “teachers have been frightened”, she says.

Vine, who says she was “crushed” by her years in teaching, has found herself a job as an education adviser at a local authority. “I’d have cut my tongue out rather than say this when I started,” she says, “but the only way I’d ever teach again is in a private school, because they’re not under the same pressures.”

Dr Emma Kell, a senior leader at a north London school and the author of the book How to Survive in Teaching, says there are things teachers can do to ease the pressure. “Be assertive with your line manager. Don’t be afraid to say no,” she advises. “The word ‘reasonable’ is very powerful, as in ‘what you’re asking me to do isn’t reasonable’.”

She also points out that teachers can be their own worst enemies. “Some of the best teachers I’ve worked with are massive perfectionists,” she says. While most concerns are valid, “sometimes you can end up a little bit blinkered. No teacher ever has to pull an all-nighter. You have to be ruthless [and think] ‘the world is not going to end if I don’t finish that marking.’”

If none of that works, then rather than leaving, it is worth giving a different school a go, Kell says. “There are good places and good people out there.”

•Some names and details have been changed