A "perfect storm" of pressures could be looming for secondary schools as headteachers warn that the education system is facing the "real risk it will break".
According to the National Association of Head Teachers (Naht) budget cuts, changes to exams and problems recruiting teachers are all causing major upheaval and "loading more uncertainty onto the secondary system than ever before".
Theresa May has said the Conservative school policy will help create a place at a good school for every child and suggested most children's school choices are determined by where they live or their parents' wealth.
Speaking ahead of the Naht's annual conference general secretary Russell Hobby said the challenges facing secondary schools were unprecedented.
He said: "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 also warned that schools are facing "unacceptable levels of financial pressure" that headteachers fear could mean school budgets will be unsustainable in two years' time.
Ministers have argued school funding is at record levels, and stated this funding will increase as pupil numbers rise.