Education Secretary Michael Gove has told head teachers the pace of school reform in England needs to accelerate.
Speaking to the Association of School and College Leaders, Mr Gove said: "Over the next ten years the world we inhabit will change massively."
He said education would need to keep pace as technology changed "how we teach and how students learn".
But ASCL general secretary, Brian Lightman, said he feared schools could not take an accelerated pace of change.
Mr Gove told head teachers meeting in Birmingham: "Lest anyone think we have reached a point where we should slacken the pace of reform - let me reassure them - we have to accelerate".
He said the education system should equip young people for all the challenges and opportunities of the changing world.
"Over the next ten years the world we inhabit will change massively," he said.
"Technology will change out of recognition, millions more will go to university, the number of low-skilled jobs will fall, with more reward for those with good qualifications."
Afterwards, from the conference stage, Mr Lightman told Mr Gove: "I worry about your wish to accelerate reform.
"We all know as leaders that we need to consider one very important issue before we implement any reform in our own schools, and that's capacity.
"I think there's a real issue of capacity in lots of our schools, and actually, by accelerating the reform too much, you actually prevent us from doing it properly."
Mr Gove told the conference that heads were key to implementing change and to the overall success of the education system.
He praised several head teachers and schools for their outstanding achievements and said: "We have the best generation of young teachers ever in our schools. We have the best generation of heads ever in our schools. And our whole school system is good, with many outstanding features."
But he added: "We have for generations failed to stretch every child to the limit of their ability - and we have for all our lifetimes, failed the poorest most of all."
He said there was a lack of "rooted determination in some quarters to make all schools excellent".
In questions to Mr Gove after the speech, the reaction was muted. A head from Crewe, Martin Kerridge, said the effect of constant announcements and reviews from government was of accelerating pace and uncertain direction.
Another head spoke of sustained pay freezes and negative messages from government leading to low staff morale.
In response to a question, Mr Gove hinted he would be in favour of a relaxation of Ofsted's plan for no-notice inspections.
England's schools inspectors had been planning on moving to no-notice inspections because of claims that they do not get a true picture of schools if notice is given.
"I know that there is a lot to be said for the principle that if you are going to have an inspection at no notice then it should actually be at short notice," Mr Gove said.
"I think a head teacher would want to be in their school in order to greet the inspector; I think they would like to know the afternoon or evening before."
He added: "I'm leaving that judgment to the chief inspector, because he's independent," which drew laughter from head teachers.
Mr Gove also said that a poll by the union and the Times Educational Supplement, which suggested that half of heads were considering quitting, had given him pause for thought.
However, speaking to journalists outside the conference hall, Mr Gove said the poll had not reached a majority of school leaders.
And his response to the union's concerns about the pace of change was robust.
"If people say 'It's all just a bit too much', my view is 'man up!'", he said.