Some schools in a spot survey carried out by the Centre for Residential Child Care at Strathclyde University had not been inspected for 16 years and the shortest inspection gap was six years.
The study was commissioned by Roger Kent, former director of social work in Lothian, as part of his Scottish Office-directed review of protection for the 10,000 children living away from home. Mr Kent wants HMI to inspect boarding schools every three years.
Slipping through the net?: A study of issues of safeguarding in the inspection process, by Wendy Anderson, Judy Furnivall and Meg Lindsay, admits inspections are "enviably thorough" and "extremely intensive". But it adds: "This results in a very low strike rate for completed inspections, which in the case of inspecting for safety is a risk. Further, the lack of clear public standards to inspect against means that the consistency in terms of the inspection on abuse-related issues is in doubt."
A Scottish Office spokesman said certain schools were inspected at least once a year by local authority social work teams. Independent residential schools had pupils inspected on a five-year cycle.
The Strathclyde report found that HMIs were primarily concerned with educational and curricular matters and were less thorough in examining the welfare of residential pupils. As a result of the Children Scotland Act 1995 and the increased focus on child protection, inspections will "require a change of emphasis".
The authors conclude: "Inspection cannot be expected to prevent abuse from ever occurring, but inspection can make its occurrence very difficult and inspection can make it impossible for it to flourish and spread within the residential childcare system."
Detailed examination of HMI reports found no mention of the term "abuse" in boarding schools. In residential schools, two reports spoke of "allegations" and three indicated inspections followed concerns for the welfare of children.
Investigations of six "welfare of residential pupils" reports, now a mandatory part of inspections, revealed no clear evidence inspectors had spoken to young people.
Coverage of welfare aspects was "patchy and thin", leaving the future of HMI's role in such matters "in doubt". Social work inspectors may be preferable if a school is already registered with the local authority, the authors suggest.
One headteacher warned that HMIs were "out of their depth". Another stated:
"It would have been quite easy for me not to be doing my best and still have got away with it."
The researchers say: "In residential special schools there was more emphasis on dealing with bullying, aggression, recording of incidents, restraint and difficult behaviour than on clear steps on combating abuse."
Unusual comments included "the need to deal with vermin", "the need to ensure girls were not placed under too much academic pressure" and the "need for relationships with the house mistress to be less formal". Such remarks were not found in mainstream inspections.