The headteacher of an "outstanding" school in Radlett said the classification is testament to the efforts of all the staff, pupils and parents.
Jo Galloway’s comments come after an Ofsted report rated Radlett Lodge School’s residential provision as "outstanding" for a second time.
Inspectors from the education watchdog visited the Harper Lane school, which teaches four to 19-year-olds, on March 13.
The school was awarded "outstanding" marks in all four inspection categories, which included the outcomes for residential pupils, the quality of residential provision and care, residential pupils’ safety and leadership and management of the residential provision.
In their report, inspectors said provision was outstanding because of the "extremely personalised" care provided and effective use of communication aids, which ensures young people are able to influence care.
In addition, the report said staff at all levels have the skills, knowledge and ability to protect residential pupils and promote their welfare.
The independent special school supports 55 pupils with autism, and caters for up to 14 pupils on a weekly and termly basis.
Jo Galloway, headteacher of NAS Radlett Lodge School, said the school work hard for the pupils.
She said: "It’s testament to the effort of all our staff, pupils and parents that Ofsted have recognised the school’s residential provision as outstanding.
"We work hard to make sure that the children in our care have the best education, the best opportunities and the best start in life."
Inspectors gave the school three hours’ notice prior to the inspection, and on arrival observed residential routines and practices, took part in discussions with external professionals involved in the care of pupils, and met with staff and residential pupils.
Inspectors noted the "constructive friendships" residential pupils formed with staff and other pupils.
The report said: "Because of their residential experience, residential pupils gain confidence in their abilities, build positive self-esteem, and develop new skills."
The last residential inspection was in 2011. The last academic inspection at the school also rated the school outstanding last year.
Following the Ofsted success, Radlett Lodge was visited by the mayor, Councillor Annie Brewster, who met with pupils and was given a tour of the school.
Miss Galloway continued: "It was a pleasure to welcome the Mayor to meet our pupils and see our school in action.
"We’re excited to be celebrating the school’s 40th anniversary in September and we’re currently planning lots of activities to mark this milestone."
Inspectors noted that further improvement could made if the school considered the ways the accommodation could meet the needs of all the residential pupils, especially relating to the size of bedrooms, the wider age range of residential pupils cared.