Ofsted was an organisation that inspected schools.
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. According to Clara Oswald, all teachers felt a sense of unutterable horror whenever Ofsted was mentioned. (COMIC: Blood and Ice)
Behind the scenes Edit
In the real world, it's short for for "Office for Standards in Education, Children's Services and Skills".