Ofsted has always had huge influence over schools, with both reputations and jobs at serious risk if an inspection report is bad. But changes proposed last week by its new chief inspector Sir Michael Wilshaw could see the watchdog become more powerful than ever in two ways that have received little publicity.
First, any school receiving the bottom two of Ofsted's four ratings - grades 3 and 4 - will be legally eligible for ministerial intervention, and an order to convert to an academy if this proposal is introduced from September. Previously, intervention only applied to grade 4 schools, so the change has the potential to raise the inspection stakes for many schools.
The second change relates to the proposed requirement for all schools to provide anonymised information on teachers' most recent performance management outcomes, to help inspectors judge school leadership. To understand the full implications of this proposal, it is necessary to consider comments from Sir Michael that accompanied the launch of the official consultation. He wants inspectors to be "more robust" and said that it was "quite legitimate" for them to ask heads "How many of your staff have moved up the main (pay) scale", "How many have gone through the (upper pay) threshold?" and whether a school was "providing value for money".
The last time he looked, Sir Michael added, more than 90 per cent of teachers who applied were going through the threshold, yet 40 per cent of teaching was "not good enough".
So is Ofsted about to start limiting teachers' pay rises as well as judging schools? When Sir Michael first raised salaries as an issue in November, Chris Keates, general secretary of teaching union the NASUWT, was swift to point out that the watchdog had "no remit for pay". This is true - there is no explicit remit. But perhaps the inspectorate can have an influence on teachers' pay through its judgements on leadership.
Sir Michael's clear message is that heads who give large numbers of teachers pay rises that the quality of teaching does not merit can expect to be marked down. That alone ought to be enough to prompt heads to at least re-examine teacher pay rates, according to those who advise them.
Malcolm Trobe, deputy general secretary of the Association of School and College Leaders, thinks Ofsted's stance will "bring a sharper edge to pay judgements". And Nigel Middleton from Educate Services, who runs performance management courses for heads, believes that Sir Michael's words will "frighten" them into looking again at how they deal with teachers' pay progression.
But that is not the same as saying they will be able to make changes. As Mr Trobe notes, although Ofsted is putting heads in "an extremely difficult position", most still have to adhere to the School Teachers' Pay and Conditions Document. The document states that all that is needed for a teacher to gain a rise on the main scale is "satisfactory" performance. Mr Middleton says that, in practice, this means automatic pay rises in most areas. He adds that the situation is not very different for the upper pay scale.
Mr Trobe questions Ofsted's ability to make any criticism of a school's performance management or teacher pay decisions stand up. He says heads' judgements have been made on the basis of sustained observation and monitoring of teachers. If the watchdog tries to contradict these through the snapshot impressions gained during inspections, it will leave itself open to "challenges all over the place", Mr Trobe argues.
Martin Freedman, head of pay and conditions at teaching union ATL, makes the same point and says that Ofsted is trying to link things that are just not the same. But the inspectorate has not proposed dissecting individual decisions on teachers; it just wants to use them "to look for a link between earnings and what is being delivered for pupils" - in Sir Michael's words, to search for "value for money".
Even so, Mr Trobe warns that Ofsted is getting into "a very dangerous and difficult area", because value for money is no longer part of its framework and it has no criteria to base such a judgement on. But Mr Freedman believes that Ofsted's new position does represent a "genuine risk" to his members' interests. And Mr Middleton thinks that new teaching standards, also being introduced in September, could give heads the scope to limit pay rises if they wanted.
So heads facing inspections could find themselves trapped between classroom unions fiercely resisting any attempts to clamp down on pay and the prospect of Ofsted condemnation if they do not. But if their school receives a grade 3 judgement, it may end up not having to follow national teacher pay policy after all - it could be converted into an academy.
Asked whether ministers would use Ofsted's changes to accelerate their academies revolution, the Department for Education would only say: "Intervention is designed to turn around the worst performing schools. We await the findings of Ofsted's consultation."
Sir Michael Wilshaw is proposing "a sort of national service for outstanding heads".
"These 'conscripts' will join Ofsted on a small number of inspections a year to ensure consistency of judgement," the chief inspector announced last week.
Heads could not complain about variations in judgements unless they were prepared to add their expertise to inspection. "Ofsted needs you," Sir Michael said. "Your country needs you."