Teachers raised the prospect on Thursday of taking strike action over attempts to shorten holidays and lengthen the school day.
The National Union of Teachers (NUT) suggested that in many areas, schools are beginning to move to different holiday patterns, such as changing term times.
It said it believed that part of the motivation for these changes was to keep children in school longer, cutting the need for childcare.
The union claimed that in Nottingham there are proposals to introduce a five-term school year, with the summer holidays cut from six to four weeks.
Members of the NUT in the city are set to strike over the plans next week.
Under the current system, state schools in England usually have three terms, with two week breaks at Christmas and Easter, six weeks off in summer and one week half-term holidays in February and October.
Those in favour of shortening holidays argue that pupils can forget some of what they have learnt during a long break.
It has also been suggested that changing term times could help families to avoid the price hikes on holidays that occur during the summer break.
Members of the NUT are due to debate a motion on the issue at their annual conference next month.
The resolution says: "Conference is concerned that many proposals to change the working day and year are based on very spurious evidence, and it refutes the misconception that more teaching automatically leads to more learning.
"Conference believes that the motivation behind some of the proposed changes is to keep children in school for longer and therefore reduce the need for childcare."
It adds: "Conference is well aware of the long hours already worked by teachers and the essential need for a period of genuine rest and recuperation only found by many in the long summer break."
The motion calls on the NUT to investigate the link between school holiday patterns and the academic, emotional and social development of pupils in other countries.
It also says that the union's executive should approve industrial action, including strike action, in places where negotiations to resist imposed changes to term times fail.
NUT general secretary Christine Blower said: "Teachers and pupils in England and Wales already spend longer teaching and learning than many other jurisdictions."
Evidence from a Unicef study showed that UK children are among the unhappiest in the world, she said.
"That isn't because they want to be in the classroom for a lot longer."
Ms Blower added: "You can't solve the problem of cuts to youth services by saying schools have to be open longer.
"Children have a right to do other things other than just being in school."
Education Secretary Michael Gove has previously said he is in favour of extending the school day, and potentially cutting short the summer holidays.
The Government's new free schools, as well as academies, which are semi-independent state schools, have the freedom to set their own school day as well as the length of terms.