Schools in England which open outside of school hours have a positive impact on the community - but improved results are not guaranteed, research suggests.
The researchers say extra-curricular activities are highly beneficial
According to a report by Newcastle and Manchester universities, extended services generally help improve achievement and reduce exclusions.
However, it cautioned that it was too early to predict better exam results.
Meanwhile, the Department for Education said it had exceeded its aim for 2,500 extended schools by September 2006.
According to the report, produced for the Department for Education, there was evidence of results improving.
But they warned that it was unclear whether these improvements were due to some schools taking on more children from middle-class homes with higher aspirations.
The academics warned that in some of the case study schools average achievement levels had declined during the initiative.
Ministers want all schools to offer access to extended services by 2010, with half of primary schools and one-third of secondary schools doing so by 2008.
More than 3,000 are now offering extended services and the government said it hoped the latest report would give more schools the incentive to open out-of-hours.
The move is part of a wider pledge to give pupils in the state sector the same opportunities as those at private school.
The longer school day is also hoped to help working parents.
Extended schools are intended to offer "wraparound" childcare on the school site or through other local providers from 8am to 6pm all year round with homework clubs and other extra-curricular activities available.
Many also provide access to sports, arts and ICT facilities for the wider community.
However, some critics warn the longer school day will put too much pressure on children.
Campaign for Real Education chairman Nick Seaton said: "I would think most sensible parents will think that children go to school to learn English, maths and science and that schools aren't intended to provide childcare.
"It seems an exceptionally long school day and while many children enjoy school a lot of children go to school but would like a break."
'Heart of the community'
Beverley Hughes, the minister for children, young people and families, suggested that such extended services would help "put schools at the heart of their communities".
She said the research, which echoes findings from education watchdog Ofsted, endorses the government's view that extended schools benefit not only children but also their families and the wider community.
A Department for Education spokesman said: "As the report clearly states, there is good evidence that extended schools can have significant positive effects on children, adults and families.
"They can also be associated with benefits for schools in terms of improvements on performance measures such as student attainment and exclusion rates."
Teachers' union the Association of Teachers and Lecturers said it supported extending school services but it did have concerns.
Its general-secretary, Mary Bousted, said funding was currently too fragmented and there were concerns about how to ensure children from deprived backgrounds got most attention.
She said the solution was to give local authorities the leading role to take the pressure off school staff.
She added: "This important programme will not be successful if staffing is not properly allocated and resourced, with the current school staff just expected to work longer and harder."