Education has been forced into a "straitjacket" by funding cuts, a teacher trade union leader has claimed.
The squeeze on school budgets is leaving children stressed, with mental health issues increasing, while the curriculum has been narrowed to focus on English, maths and science, the NEU's (ATL branch) annual conference in Liverpool has heard.
Josie Whiteley, president of AMiE, ATL's leadership section, told delegates that school leaders are finding themselves "being cornered into regularly having to make impossible decisions".
Ministers have insisted that there is more money going into schools.
Speaking in a debate on school funding, Ms Whiteley highlighted the impact of funding pressures on students.
"Let's consider what's happening in a few areas where funding cuts are now forcing even the most ethical of leaders to make decisions to the detriment of their students," she said.
"Larger class sizes, a narrowed curriculum, fewer support staff, fewer teachers, a lack of resources from computers to pencils, a loss of enrichment opportunities, fewer teaching hours, loss of school and college trips, lack of additional qualifications for the most able students, loss of specific support for learners with additional needs and so on.
"School budget cuts and post-16 underfunding over the last decade has led to education being forced into an absolute straitjacket with a tight focus on English, maths and science whilst wider curriculum opportunities are ignored.
'Children are so stressed'
"Schoolchildren are so stressed and, as a result, the levels of depression, self-harm and other mental health issues are increasing.
"The idea of having an Easter holiday if you are in Year 11 is just a dream because you find you have to attend revision sessions run by exhausted school staff forgetting that they actually need a break, too."
Ms Whiteley said that there is "a glimmer of hope" because "education leaders have started speaking out".
"We need to encourage them to continue to do this – to be brave, to be honest with their governors and to mobilise parents explaining to them why enough is enough," she said.
The union passed a resolution which raised concerns that "low levels of funding for education are forcing many leaders to implement unethical changes in schools and colleges to the detriment of learners".
It calls for the executive to take action, including ensuring that school leaders and managers are reassured that they will be supported by the union and encouraged to speak out and "challenge the funding barriers they face".
A Department for Education spokesperson said: "Thanks to our reforms and the hard work of teachers, there are 1.9 million more children in a 'good' or 'outstanding' school than in 2010.
"By 2020, core school funding will rise to a record £43.5 billion – the highest ever and 50 per cent more per pupil in real terms than in 2000 – and the introduction of the national funding formula will address historic disparities in the system."