So many families come and go that teachers spend a lot of time assimilating new pupils. The high turnover can reflect poorly on achievement and stretch budgets.
This is one reason cited for the authority's poor performance in the latest value-added figures for pupil achievement, which consider deprivation in relation to entitlement to free school meals (FSMs).
But many believe poor achievement is down to funding. Suzanne Nantcurvis, NASUWT Cymru national executive member for North Wales, said: "It's not a question of how can we improve next year - it's how can we make ends meet without losing teachers?"
Meurig Rees, head of Ysgol Glan Clwyd in St Asaph, said: "If the authority was given the funding of more favourable authorities, or of those in England, we would be far better-placed to address the drop in standards."
The authority's head of education services, Ieuan Lloyd-Roberts, recognises money is a problem, but is keen to stress other factors.
He says transience is a major issue, while the geography of the authority features urban coastal areas containing pockets of deprivation and the rural hinterland.
Rhyl, home to two of the authority's eight secondary schools, "is the main area causing concern", said Mr Lloyd-Roberts, who questions the reliability of FSMs as an indicator of deprivation.
Rhyl high school takes pupils from one of Wales's most deprived wards but because it does not have the facilities to cater for large numbers of children at lunchtime, many go home. This has left the school with a lower FSM figure of around 19 per cent, which puts it in a higher benchmarking group. It also means it does not qualify for the Assembly government's new pound;16m Raise fund boosting the attainment of poorer children.
"There's a lot of concern among the North Wales authorities that relying on FSMs alone does not reflect performance and deprivation," said Mr Lloyd-Roberts.
Taking into account FSMs, Assembly government statisticians predicted that 62 per cent of Denbighshire 14-year-olds should have reached the standard expected for their age in EnglishWelsh, maths and science in last summer's tests. In fact, only 55 per cent did so.
It was the worst result in Wales at KS3, and the authority was second worst at GCSE after Wrexham. The prediction was that 56 per cent of Year 11 pupils should get five good passes, but only half did. Denbighshire was 21st out of 22 LEAs in terms of gross expenditure per secondary pupil delegated to schools in 2005-06. Pupils had pound;800 less spent on them than their counterparts in Ceredigion, the highest-spending authority.
Mr Lloyd-Roberts has squeezed an extra pound;470,000 from councillors for KS3 this year and emphasises the authority's use of Fischer Family Trust assessments of pupil abilities and predicted results.
There is a school improvement group where deputy heads meet to share good practice. Schools are encouraged to focus on marginal pupils on the borders of C-grade GCSEs, and on transition between primary and secondary school.
But transience can undermine this work. Of those who left Christ Church primary school in Rhyl last year, just a third had started the school in the nursery.
Chris Thompson, headteacher, said: "You can never gauge it, or plan what the numbers will be, because families move in and out. Some stay longer than others and they all have their own domestic needs."
For details see new.wales.gov.uktopicsstatistics