A freedom of information request from the Scottish Conservatives showed that out of the country’s 2,372 state schools, 981 were last assessed by Education Scotland before the start of 2012.
Two schools in Aberdeenshire were found to have had their last assessment 18 years ago, in 2004.
Inspections in schools were impacted by the coronavirus pandemic, but the findings suggest targets were being missed beforehand, with 2018’s total of 250 assessments falling short by 59 schools.
Inspections have not resumed since their pause in 2020, with the Scottish Conservatives accusing the SNP and Education Scotland of “hiding behind the pandemic”.
Scottish Conservative education spokesman Oliver Mundell said: “These figures are deeply concerning for parents, pupils and teachers.
“We need a rigorous, reliable and, above all, regular inspections system so that parents are aware of how their child’s school is performing and so that schools know what they need to do to improve what they are offering pupils.
“It’s absolutely appalling and completely unacceptable that two schools haven’t been inspected since way back in 2004. But sadly, it’s in keeping with the long litany of education failures the SNP have presided over.
“The pandemic has clearly not helped but it does not excuse the decline in inspection rates prior to Covid or the SNP’s failure to meet its own targets on this.
“I’m pleased that the SNP Government finally listened to the Scottish Conservatives and recognised that it was wrong for Education Scotland to both govern schools and carry out inspections.
“But the SNP and Education Scotland have hidden behind the pandemic for too long. We need the inspections programme to restart urgently, to get on top of this growing backlog, and we need an independent inspectorate to carry out school assessments, to ensure transparency and trust in the process.”
It comes after MSPs heard the recovery of Scotland's education system from the pandemic should aim to improve on pre-Covid levels.
Andrea Bradley, assistant secretary of the education trade union the EIS, told the Education, Children and Young People Committee at Holyrood on Wednesday that it would take "creativity, collaboration and additional resource" to recover from the pandemic.