Councils have been under Scottish Executive pressure for the past two years to do far more for looked-after children but policies have yet to translate into leaving results. Some 37 per cent of leavers managed passes in English and maths, with 237 achieving at least one pass at Standard grade.

Success rates vary across the country. Highland had more than 70 per cent achieving Standard grades in English and maths against 20 per cent in North Lanarkshire.

This is the first time information on the educational attainment of looked-after children has been collected by the public spending watchdog.

Among other school performance statistics, Audit Scotland, which reports to the Accounts Commission, found that just under a third of primaries were less than 60 per cent full. Four authorities - Argyll and Bute, Dundee, the Western Isles and Glasgow - have at least half their primaries under the recognised standard.

East Ayrshire, Inverclyde, Moray and South Ayrshire share problems with at least 40 per cent under-occupancy.

In contrast, Aberdeenshire, Perth and Kinross and West Lothian report that more than one in five primaries were over-occupied in 2001-2002. Edinburgh is overflowing in almost one in five.

Brian Monteith, Tory education spokesman, accused the SNP of misleading the electorate into believing class sizes could be cut to 18 when key authorities were “full to bursting with class sizes of 30”. A massive amount of extra capital investment would be needed to build new classrooms under the SNP’s policy, Mr Monteith said.

Audit Scotland also confirms that authorities had under 1 per cent of P1-P3 classes with more than 30 pupils in 2001-2002, against 7.2 per cent the previous year.