There are no fixed rules for deciding which schools can become academies, the Department for Education and Skills admitted this week.
A DFES spokesman said proposals were considered on a case-by-case basis.
"There are no hard and fast rules," he said. "There are many factors that are looked at." These included a school's performance at GCSE, inspection reports, its popularity with parents, the proportion of pupils receiving free school meals and levels of special needs.
The sheer variety of schools replaced so far underline the scope that the DfES has to approve proposals. Three thriving city technology colleges are included, as are others inspectors said were doing a good job in difficult circumstances.
Ministers' tendency to characterise the replaced schools as "failing" has left their former staff seething. Gordon Potter, former deputy head at Coulby Newham school, Middlesbrough, which was never failing, said: "I remain angry every time I hear the work of my former school being rubbished."
Gill Reed, former deputy head of science at Willesden high, Brent, north London, said: "Our school achieved real successes against the odds."