A report from the spending watchdog, out on Tuesday last week, showed although schools were spending about £21bn a year on training teachers there remained a problem with staff retention.
Amyas Morse, head of NAO, said: “Schools are facing real challenges in retaining and developing their teachers, with growing pupil numbers and tighter budgets.
“The trends over time and variation between schools are concerning, and there is a risk that the pressure on teachers will grow.”
The NAO noted government spending on training and supporting new teachers went down from £555m in 2013/14 to £35.7m in 2016-17 on programmes for teacher development and retention, of which £91,000 was aimed at improving teacher retention.
Growing workloads were cited as an issue for the sector, the watchdog NAO, and that in 2016 34,910 teachers (8.1% of the qualified workforce) left for reasons other than retirement.
In an NAO teacher survey, 67% of respondents reported that workload is a barrier to retention.
A Department for Education survey found classroom teachers and middle leaders worked, on average, 54.4 hours during the reference week in March 2016, including the weekend.
The loss of some existing staff comes against the background of an overall increase in the number of teachers in state-funded schools in England which went up by 15,500 (3.5%) between 2010 and 2016, reaching 457,300 in total last year.
But the number of secondary school teachers fell by 10,800 (4.9%) over the same period as these schools face significant challenges to keep pace with rising pupil numbers.
Schools filled only half of their vacancies with teachers with the experience and expertise required and, in around a tenth of cases, schools did not fill the vacancy at all, an NAO survey of school leaders found.
Although, the greater number of qualified teachers are returning to state-funded schools, and the NAO have identified that the DfE and schools have scope to attract back even more teachers who have left and benefit from the investment made in their training.
In 2016, 14,200 teachers returned to work in state-funded schools, an increase of 1,110 on 2011, the NAO noted.
The report said the DfE still lacks data on local supply and demand and cannot show that its interventions are improving teacher retention, deployment and quality.
Geoff Barton, general secretary of the Association of School and College Leaders, said the teaching profession had been “drained of joy” by endless government reforms, severe underfunding and a lack of accountability.
“This has resulted in unsustainable workloads at all levels and we desperately need a period of stability free from further reforms,” he said.
Ministers must address the issue of teachers' pay, Barton believed.
A DfE spokesperson said: “There are now a record number of teachers in our schools – 15,500 more than in 2010 - and overall the number of new teachers entering our classrooms outnumbers those who retire or leave.
The government said it had set up a £75m fund to support teacher development in schools where retention is an issue.
