Ofsted

Ofsted was an organisation that inspected schools. According to Clara Oswald, all teachers felt a sense of unutterable horror whenever Ofsted was mentioned. (COMIC: Blood and Ice) Indeed, Frank Armitage said of Paul Smith, "He's from Ofsted. Of course he's evil." (TV: The Coach with the Dragon Tattoo) In 1580, Amy Pond mocked Rosanna Calvierri when the latter tried forcing her into revealing her true identity by claiming she was from Ofsted. Rosanna's reaction suggested that she knew who Ofsted were despite them not having been formed yet. (TV: The Vampires of Venice)

In 2048, the Twelfth Doctor used his psychic paper to pretend to be an Ofsted inspector in order to investigate Snowcap University. (COMIC: Blood and Ice)

The Inspector disguised himself as an inspector from Ofsted named Paul Smith, in order to evaluate Miss Quill at Coal Hill Academy. (TV: The Coach with the Dragon Tattoo)

Professor Ernest Fleetward asked the Eighth Doctor if he was from OFSTED. (PROSE: Sometime Never...)

When the Tenth Doctor showed his psychic paper to a group of children on Arcopolis, it said that he was from the planet Ofsted, which he found funny, but the children didn't get the joke. (PROSE: The Eyeless)

Behind the scenes
In the real world, it's short for "Office for Standards in Education, Children's Services and Skills".