Class-size targets for early primary have inflated the number of composite classes in one of Scotland's largest authorities - and similar trends are emerging across the country.
The surge is causing unrest among parents and coincides with rising class sizes in the upper primary years. There are fears, meanwhile, that senior staff's management time is being compromised.
The trends emerged in a paper to North Lanarkshire Council's learning and leisure services committee this week. It detailed efforts to reduce P1-2 classes to 23 pupils or fewer, in the drive towards the Scottish Government target of 18 in P1-3.
But headteachers reported that the knock-on effect of achieving this for 90 per cent of P1-2 children was that P3-7 classes became bigger and the number of composite classes grew.
A briefing paper to Murdo Maciver, head of educational provision, shows the proportion of primary composite classes throughout the authority rising from 24.5 per cent in 2006-07 to 32.5 per cent in 2009-10. The proportion of pupils in such classes rose from 22.4 to 29.6 per cent, but the council stressed children could be taught "equally effectively" in single-stage or composite groups. In the same period, the average P7 class went up from 24.7 to 26.7 pupils.
One education official told The TESS the situation resembled a tube of toothpaste: without extra funding, a squeeze in P1-2 classes inevitably produced a bulge elsewhere, and an extra two pupils in the important final year of primary school was unlikely to be welcomed.
Meanwhile, the council will investigate predictions from a quarter of its primary heads that, in 2009-10, smaller P1-3 classes would require senior staff to teach more often.
Judith Gillespie, development manager of the Scottish Parent Teacher Council, said parents were becoming "really agitated" about increasing composite classes in urban schools, which the Educational Institute of Scotland confirmed was a "growing issue" across the country.
While composites in rural schools kept year-groups together, urban schools left some pupils in a single-stage group as others joined a different stage. Those separated from peers suffered a "nightmare" of stigmatisation, being branded "stupid", and feeling like outsiders in classes which were already strong units, said Mrs Gillespie.
Greg Dempster, general secretary of the Association of Headteachers and Deputes in Scotland, warned that, without extra funding, a repeat of North Lanarkshire's trends was inevitable wherever P1-3 classes were being driven down.
A growing number of composite classes was not ideal, but did not in itself cause him too much concern. More problematic was the anxiety created in schools unaccustomed to composite groupings and among parents, who disliked mixed-age classes.
The benefits of smaller P1-3 classes made it worthwhile pursuing the Government targets, Mr Dempster said, especially since there was less evidence that smaller classes benefited older children.
But "all sorts of administrative headaches" were looming, he warned, if pupils moved from smaller to larger classes in progressing through primary school, and back to smaller classes in secondary school.
A Scottish Government spokeswoman stressed that average primary classes reached a record low of 23.2 pupils in 2008, and that a review of class- size regulations had been announced earlier this month.