Secondary schools are facing significant challenges in recruiting enough teachers to keep up with rising pupil numbers, a Government spending watchdog has warned.
Tens of thousands of teachers left England's schools before reaching retirement age last year and headteachers are finding it difficult to fill jobs with good-quality candidates, according to a new report by the National Audit Office (NAO).
It concludes the Department for Education (DfE) cannot show its attempts to keep teachers in the classroom are having a positive impact and are good value for money.
The study says almost 35,000 qualified teachers (34,910) left the profession for reasons other than retirement last year.
While overall there was a 13.2% increase in the number of primary and nursery school teachers between November 2010 and the same point in 2016 - 26,000 extra workers - during the same period there was a 4.9% fall (10,800 staff) in the numbers of secondary-school teachers.
A survey conducted by the NAO found 85% of secondary-school leaders did not think they had been given enough support by the Government to retain high-quality teachers while 67% said that workload is still a barrier to keeping teachers in the profession.
The survey also found schools only filled half of their vacancies with teachers that had the right experience and expertise, and in around one in 10 cases the post was not filled.
The NAO also says DfE initiatives to support the teaching workforce have been "relatively small scale", estimating the department spent £35.7 million in 2016/17 on teacher development and retention as well as an estimated £34.2 million on schemes aimed at improving teacher quality.
In comparison, in 2013/14 £555 million was spent on training and supporting new teachers.
The study did find that more qualified teachers are returning to state schools, with 14,200 heading back into the classroom last year, up 1,110 on 2011.
The watchdog concludes: "Having enough high-quality teachers in the right places is crucial to the success of the school system and to securing value for money for the £21 billion that schools spend on their teaching workforce.
"Performance against national indicators suggests progress: the overall teaching workforce has been growing and more children are in schools where Ofsted has rated teaching, learning and assessment as good or outstanding.
"These indicators, however, mask significant variation between schools and concerning trends, especially in secondary schools.
"Schools are facing real challenges in retaining and developing their teachers, particularly when they are also expected to make significant savings by using staff more efficiently."
NAO chief Amyas Morse said: "Schools are facing real challenges in retaining and developing their teachers, with growing pupil numbers and tighter budgets.
"Since having enough high-quality teachers is essential to the effective operation of the school system, these are issues that the Department needs to address urgently."
A DfE spokeswoman said there are 15,500 more teachers in schools than in 2010 and "significant sums" are being spent on teacher recruitment.
"We recognise there are challenges facing schools and we are taking significant steps to address them," she said.
"We have established a £75 million fund to support high-quality professional development in those schools where teacher retention is an issue, and we are making it easier to advertise vacancies."