There is a "serious danger" that severe budget cuts will mean that schools will not be able to maintain their current standards of education, headteachers have warned.
Schools across England are being forced to cut courses, equipment and books, increase class sizes and make redundancies amid a continuing squeeze on finances, according to the Association of School and College Leaders (ASCL).
It accused the Government of "asking the impossible" by demanding that schools improve standards, without giving them the money and resources to do so.
A survey of ASCL members, conducted by the union ahead of its annual conference in Birmingham on Friday, found that more than three in four (77.1%) believe financial pressures have had a detrimental impact on the education they are able to give pupils.
The vast majority (84.6%) said they have not received enough funding in the past 12 months to meet the essential needs of their school or college.
More than half (52.7%) said their financial situation was serious, or very serious, with a further 17.6% admitting it was critical. More than one in four (28.3%) said their situation was worrying but that they were coping, and the rest said their situation was fine.
Nearly seven in 10 of those polled (69.5%) said they had been forced to cut resources such as IT equipment and textbooks, while 65.9% said they had reduced the number of courses they offered and 63.9% had increased class sizes.
More than a third (38.4%) had made job cuts. Others said they had cut senior leadership posts or not replaced departing staff.
The survey also found concerns that the situation is unlikely to improve, with a third (33.5%) saying it will be "critical" and a further 56% saying it will be serious, or very serious.
Research has shown that England's schools will have to make real-terms cuts of around 8% over the next five years, because of extra costs that they will have to meet from their existing budgets, ASCL said.
It added that according to its own analysis, a secondary school teaching 1,700 pupils aged 11 to 18 with a budget of £7.9 million would have to make cuts worth £531,000 in 2016/17 - equivalent to more than 10 teachers.
In his speech to the conference on Friday, ASCL president Allan Foulds is expected to say that the combination of funding cuts with a teacher shortage poses a serious threat to schools and colleges.
"These problems are so acute that there is a serious danger we will not be able maintain current standards, let alone raise them further.
"Geoffrey Howe once made a famous observation about being sent to the crease only to find that your bat has been broken by the team captain. The situation with teacher supply and funding is now so serious that we are in danger of finding we are out there with no bat at all.
"The Government is rightly committed to raising standards. Nothing is more important to school and college leaders. But it is simply asking the impossible to demand that schools and colleges take the next big leap forward in raising the bar without providing the essential materials with which to achieve that ambition.
"There is a simple correlation between input and output in any process. The education system requires the raw materials of sufficient funding and teacher supply to achieve the outcomes we all want to see.
"No amount of hard work and dedication in schools and colleges can make up for the lack of them and it is wishful thinking to believe otherwise."
A separate report published by ASCL and the Policy Exchange think-tank on Friday argues that there should be more flexibility to allow teachers to come in and out of the classroom throughout their careers.
It argues that between 2008 and 2012, 6,000 women aged between 30 and 39 left the profession each year, and for many this is likely to be related to taking time out to care for children. Separate data shows that of teachers who leave in order to look after family, only around half will return to the classroom.
At the same time, of all the teachers who leave state school each year, around half stay in education in some form, the report says.
It suggests that more flexible working, including career breaks and secondments, could encourage more people to stay in teaching.