Officials are investigating why primary pupils in Neath Port Talbot are more likely to be absent on Mondays and Fridays. Attendance in the authority's primaries was well below the Welsh national average last academic year.
Estyn inspected the authority in June and is targeting schools with the worst records. There has seen a slight improvement this year, but the authority is still keen to find out why there are so many days off at the beginning and end of the week, which is more in line with industry than schools.
"There does seem to be a problem on a Monday and a Friday," said Aled Evans, Neath Port Talbot's head of education development and inclusion. "We've begun to pick and support primaries where there are issues, but it may be that attendance is related to a child's wellbeing."
In its report published last week, Estyn awarded the authority grade 1, four grade 2s and one grade 3. Its prospects of improvement were also said to be good. But the poor performance of primaries - academically and on attendance - is cause for concern.
One of Neath Port Talbot's greatest strengths is that its pupils buck the Welsh trend at key stages 3 and 4, producing the best KS4 performance in Wales in 2007.
But Mr Evans said there was no excuse for below-par performances in primaries. Approaches to literacy are being looked at closely.
"We felt the needs of youngsters in literacy as assessed by staff at key stages 1 and 2 didn't manifest itself in a focused and targeted approach," he added. But he said Flying Start, an Assembly government initiative for under-3s, offering free childcare and support for parents, was beginning to show beneficial effects.
The authority also focuses strongly on children approaching transition to secondary school where the picture of achievement is rosier. "We do a lot of work to maintain continuous learning between Years 6 and 7 where this can be vulnerable," said Mr Evans. "We've seconded a headteacher to support those in their last two years at primary."
A-level results and numbers of pupils gaining five or more top-grade GCSEs are consistently above the Welsh average.
Estyn also praised heads in Neath Port Talbot for their active role, but says fixed-term and permanent exclusions in secondary schools are relatively high and have remained so from 2005-07. Progress in addressing it has been slow, says Estyn.
There was praise for the authority's attempts to correct bad behaviour with school nurture groups.
Estyn also praised the achievements of children who have additional learning needs. It says the authority's provision is outstanding. It also praised Neath Port Talbot's record on social inclusion - last year 91 per cent of looked-after children left school with qualifications, compared with 81.9 per cent nationally.