The most needy schools are being denied grant aid to open their gates to the wider community.
The finding, in the Estyn report Provision of Community Focused Services in Schools, comes despite research proving pupils attending out-of-hours study support sessions can raise their achievement by at least half a GCSE grade.
Inspectors call for schools to step up relations with their local community and to improve their image, but said many do so within their existing budgets.
Local authorities were criticised for not having good enough risk-assessment plans covering child protection and health and safety.
Estyn looked at how well schools and LAs were using the community focused schools (CFS) grant, monitoring five local authorities. It found few of them had targeted the most disadvantaged schools, and there were huge variations in funding.
In 2006-7, the use of funding by secondary schools in 13 local authorities varied between 20 and 63 per cent.
The CFS grant is administered by LAs and is available for projects. Funding runs until 2011.
But the report says: "CFS activities in some schools are ineffective or weak and not matched to the needs of the wider community."
It calls for the Assembly government to clarify funding and recommends that schools should work more closely with families.
A spokesperson for learning charity ContinYou, which also provides funding, said it was recognised that there was some way to go for schools to meet the broader needs of the community.