The school funding crisis is leading to cuts in teacher numbers and an increase in their workload, Ofsted warned today.
The breadth of the curriculum and provision for pupils with SEND is also being cut back because of school "funding pressures", Amanda Spielman has said.
The chief inspector said that a study of schools conducted by the watchdog to examine the impact of these pressures had found that "most" were cutting teacher numbers.
She made her comments in a blog that was published this morning, but then taken down because Ofsted said it was not ready to publish the research that accompanied it.
Spielman: Inspections can add to teacher stress
"In some of the secondary schools we visited, subject specialists were not being replaced when they left and other teachers were teaching outside their specialism," Ms Spielman writes.
"In some schools, experienced teachers were replaced with less-experienced and lower-qualified staff. Schools also reported cutting back continuous professional development and removing teaching and learning responsibility points.
"In some schools, higher level teaching assistants were being used to cover classes when teachers were absent, rather than the school paying for teachers to cover these lessons."
The chief inspector says that taken together these decisions they "may lead to a reduction in expertise in the school, with less-experienced teachers and more out-of-subject teaching".
"This becomes even more of an issue when teachers are not then supported through high-quality professional development," she added.
"Although staff cuts may be necessary, this is sometimes being done with insufficient monitoring of the effects on quality of education. This is clearly not the way to ensure that children and young people get the education they deserve."
She also noted that the average age of teachers in England was already the lowest in the OECD according to the 2018 Teaching and Learning International Study (Talis).
"Funding should be used to retain experienced staff with expert subject knowledge in schools," Ms Spielman said.
Schools that Ofsted spoke to for the research also said that school "funding pressures" were affecting teacher workload.
"Teacher-student ratios have been increasing," Ms Spielman said. "Teachers are receiving less hands-on support from senior leaders, who are having to increase their own teaching time.
"Senior leaders told us they are taking on additional work due to staff number reductions. They are also doing more outside the school to bring in income, such as working across more than one school, consultancy work or working as Ofsted inspectors.
"As we found in our teacher wellbeing study last year, this is likely to increase teacher stress and harm retention. Schools reported that this was already happening, with a greater rate of teacher absence in turn leading to more instability for pupils."
Her blog acknowledges that funding pressures mean schools are having "to make tough decisions and difficult choices".
But she warns that sometimes are schools are not considering the impact of their responses to a lack of funding.
"Schools were responding to these pressures by reducing staffing, cutting back on non-essential building maintenance, and limiting additional provision for pupils, not least those with SEND," she writes.
"In many cases, we found that schools were making these decisions in an informed way, using benchmarking and other evidence.
"However, too often we found that decision-makers were not sufficiently monitoring the impact of their decisions on the quality of education and on their most vulnerable pupils."
She said SEND provision, curriculum breadth and education quality were also affected.