Apostrophes should not be used to indicate plurals, commas should be kept to a minimum and Great Britain does not include Northern Ireland, which is part of the UK only, Ofsted's house-style guide reminds inspectors.
The guide, on the inspectorate's website, is intended to "help those writing and editing Ofsted documents to present them in a clear and consistent style so that readers can readily understand the content".
A word list to help inspectors with spelling and when to use capital letters includes such tricky items as "civil servant", "database" and "internet".
John Bangs, National Union of Teachers' head of education, said action was needed. "No government department, including Ofsted, is immune from the kind of howler that appears occasionally in children's work. Although it is right to demand schools teach apostrophes properly, it is also right to demand that Ofsted and other government departments raise the bar of what is considered satisfactory and get their own houses in order."
The guide includes instructions to inspectors to avoid stereotyping race, class, sex or age groups, in particular by avoiding the conventional use of "he" to refer to a person of either sex. Unnecessary capital letters should also be avoided, inspectors are told.