Governing bodies must get better at holding their schools to account, according to Wales's chief inspector.
Ann Keane, head of schools inspectorate Estyn, said there were shortcomings in the performance of governors in around a quarter of the secondary schools inspected in the last academic year.
Speaking at the annual conference of Governors Wales at the weekend, Ms Keane revealed that while there had been a significant improvement in the work of primary school governors over the last six years, at secondary level the performance was more variable.
The effectiveness of governors in helping to set the strategic direction of their schools had worsened, she said.
At primary level there are still shortcomings in around a third of schools, where governing bodies are not sufficiently probing achievement or benchmarking against other schools in similar circumstances.
"At secondary level some governors seem to be unaware of the shortcomings and fail to challenge to the expected level," Ms Keane said.
She told governors: "You need to probe the performance of the school. There are lots of questions you can ask."
However, Ms Keane told delegates that there have been improvements in the number of governing bodies meeting their statutory requirements in areas such as collective worship and disability discrimination.
Just 14 per cent of primary governing bodies failed to meet such requirements last year, compared with a quarter of those inspected in 200405. Just less than a quarter of secondary governing bodies had failings, compared with almost half in 200405.
Estyn will review the performance of governing bodies over the last inspection cycle in next year's annual report.
Overall, Ms Keane said three-quarters of secondary and 80 per cent of primary schools were doing an "excellent job", and in 10 per cent of primaries governors provided "outstanding" support to managers and a good level of challenge.
"Governors play such a vital role in our schools in Wales," said Ms Keane. "It's very important that governors are fully involved in the processes of self-evaluation to make sure they accurately reflect the strengths and shortcomings of their school."
Jane Morris, director of Governors Wales, said: "We recognise that there is much good practice among governing bodies but it's also true to say there are some shortcomings and areas of weakness. We would urge governing bodies to ensure they fulfil their strategic role, act as a critical friend and monitor and evaluate effectively."
Governors from across Wales attended last week's conference in Llandrindod Wells to discuss issues of school effectiveness and collaboration.
David Hopkins, former director of education at Caerphilly Council, told delegates they would have an important role to play in the attainment- raising School Effectiveness Framework (SEF), which is set to be rolled out across Wales from September.
Mr Hopkins, who is now an adviser to the Assembly government on SEF, said governors must get full access to school data so they can monitor progress and challenge headteachers.
- Original headline: Problem governors `in a quarter of secondaries', warns inspector