Secondary schools in England are facing a "perfect storm" of pressures that could have severe consequences for children, headteachers have warned.
Budget cuts, changes to exams, problems recruiting teachers and Brexit are causing major upheaval, according to the National Association of Head Teachers (NAHT).
The union is also expected to argue against Government proposals to expand grammar schools at its annual conference this weekend.
Speaking ahead of the Telford meeting, general secretary Russell Hobby said: "The combination of challenges facing secondary schools and their students has never been greater.
"Many school leaders are concerned about maintaining high standards in the face of simultaneous upheaval on so many fronts. It's a perfect storm.
"The Government is loading more uncertainty onto the secondary system than ever before. There is a real risk it will break."
Mr Hobby repeated warnings that schools are facing "unacceptable levels of financial pressure", with an NAHT survey showing that 72% of headteachers believe that school budgets will be unsustainable in two years' time.
"This is a result of the Government's choice to freeze spending and keep it at 2010 levels for each pupil. The 2010 cash isn't going as far as it used to. You can't expect it to. But the Government is flatly refusing to admit the reality."
Ministers have argued that school funding is at record levels, and that this will increase further as pupil numbers rise.
Outgoing NAHT president Kim Johnson attacked suggestions that schools need to make efficiency savings.
"It's quite insulting to have ministers say to you 'you need to renegotiate your photocopying contract, perhaps think about the paper you're getting in, club together with six other schools and you'll get it cheaper'.
He said that his school, Bradfields Academy - a special school in Kent, spends 90% of its budget on staffing.
"Changing those procurements is not going to impact in the way in which I can retain staff. I'm going to have to make redundancies."
Schools are finding it increasingly difficult to recruit and retain staff, the NAHT said, particularly in key subjects such as maths, science and languages.
"Year after year, the Government has missed its own targets for teacher recruitment," Mr Hobby said. "30% of new teachers leave the profession after five years.
"NAHT's own research shows school leaders have struggled or failed to recruit in eight out of 10 cases this year. Recruitment has never been more challenging."
This summer, schools are also dealing with exam changes - including a new GCSE grading system.
There are also concerns by some that a Government expectation that 90% of pupils will study English Baccalaureate subjects - English, maths, science, history or geography and a language, is narrowing the curriculum, the NAHT said - and uncertainty about how Brexit will impact on the thousands of EU nationals who work in schools.
The NAHT's motion on selective schools says the union should "campaign vigorously to reject the proposed expansion of selection" in the absence of "any compelling evidence that it promotes social mobility".
Theresa May has said the policy will help to create a place at a good school for every child and argued that many children's school choices are determined by where they live or their parents' wealth.