State schools should operate longer days of up to nine or ten hours, according to Education Secretary Michael Gove.
His claim came as part of a speech this week in which he said state schools in England should be as good as private schools.
But union bosses in Lancashire said longer days would only be considered if they were properly funded and supported.
Tony Roberts, Lancashire’s membership secretary for the National Association of Headteachers, said teachers already worked long hours.
He said: “A large number of schools will do pre-school activities and after school activities, so this isn’t a brand new idea they have come up with.
“If you want to follow this route of having an extended school day, you’ve got to fund it and provide for it.
“Because the way the current teacher’s day works is that in most schools you have people in from 8am or before.
“Then they all have activities and meetings and preparation after school, so we’ve not got this mass exodus at the gates at 3.30pm.
“Most teachers will work at some point at the weekend, so in other words the figure needs to take into account what’s going on on the ground anyway. But if they are going to do it, they have got to put some additional funding and support in.”
Mr Roberts said the union wouldn’t reject the idea if it was properly supported.
But he said: “It is part of this climate where if you get a new idea or somebody comes up with something, the schools will do it. Sometimes it is quite right that they do, but sometimes it is right that they do what they are paid to do which is to educate and nurture children.”
In a speech at the London Academy of Excellence, Mr Gove said he wanted to break down the “Berlin Wall” between state and independent sectors.
But Mr Roberts described the idea as “absolutely ridiculous”. He said: “If you are going to raise the standard of all state schools to private schools, then you have got to gift schools the money that the private sectors get.
“The average independent school may get two, three, even four times more money per pupil than the state school. In independent schools, you have small classes, a wide range of extra curricular activities you can afford to fund, and you don’t have interference from government.”