Head teachers have threatened to block plans to extend the school day in England, branding it a "national baby-sitting service".
Ruth Kelly wants schools to open from 8am to 6pm
General secretary of the National Association of Head Teachers Mick Brookes said it had "worrying" financial implications for schools.
The NAHT's annual conference was also told that the "loving neglect" of some parents created barriers to education.
The Department for Education said extended school services were valued.
Education Secretary Ruth Kelly wants schools to open from 8am to 6pm.
But Mr Brookes said the burden on schools could undermine the goodwill of staff.
He told delegates in Harrogate that schools were "being inveigled into the administration of the national baby-sitting service".
He continued: "The as yet unpublished guidance to schools on the financial aspects of extending schools is very worrying.
"The requirements to charge for some after-school activities and not for others, to co-ordinate community transport and to further erode financial capacity by raiding school budgets to support such schemes not only threatens administrative chaos but also the loss of goodwill that may well see the end of the traditional after-school club.
"We must have the courage to put progress on hold if it threatens to further erode the work-life balance of school leaders and therefore damage the capacity of the school to fulfil its core purpose."
Mr Brookes said later: "Condemning children to a 50-hour week - eight to six - doesn't quite fit with my idea of childcare."
He said the extended hours plan could see parents having to pay around £70 a week for their children's after-school clubs.
A spokeswoman for the Department for Education said: "Extended school services are valued in the communities they serve in, and are there for all young people, not least the most disadvantaged in our society."
By 2010 ministers want all schools to be offering a range of clubs and out of school activities in line with Ms Kelly's plan.
Mr Brookes also warned that a minority of parents were "washing their hands" of responsibility for their children.
"The vast majority of parents are supportive, concerned and well meaning.
"But there is a minority who create huge barriers to learning for their children and others by sending them to school in an unfit state to learn with negative and violent attitudes to authority, or who simply don't send them at all."
He warned that some pupils were too tired to pay attention in class because their parents failed to ensure they got to bed on time.
He said "Some of it is children staying up far too late and playing video games in their bedrooms into the early hours and coming into school too tired to work.
"They are rather like me. When I get tired, I get grumpy. You end up with children on a shorter fuse because they are exhausted."
He said he was probably talking about 5% of families and stressed that most parents he had worked with in his time as a headteacher were "fantastically supportive" of the school. But some parents "love their children too much to say no".