The clubs, which are central to government plans to extend the school day, rank at the bottom of a list of childcare providers offering a good service, according to Ofsted.
Just 44 per cent of school clubs were ranked as good or outstanding by inspectors, a dip of 1 per cent from last year.
Seven per cent of school clubs are ranked as inadequate, the same as last year. Only creches had a higher number of inadequate settings, with 8 per cent falling into the category.
The revelation, in an Ofsted report into all childcare settings, is likely to worry ministers, who have ambitious plans to expand the number of schools caring for children from 8am to 6pm.
The Government wants there to be places for half of primary children by 2008 and for all 4- to 14-year-olds by 2010.
Beverley Hughes, the children's minister, announced this week that 7,000 schools are now offering extended services, with that target being hit two months ahead of schedule.
Ofsted carried out more than 27,000 inspections of childcare and early years learning providers between April 2006 and March 2007.
There are more than 106,000 childcare settings registered in England, providing in excess of 1.5 million places for children under 8 years old.