Cyfarthfa high school, in Merthyr Tydfil, serves a community notorious for its high rates of unemployment, drug use and teenage pregnancy.
But Estyn, the inspection body of the Welsh Assembly, has found that Cyfarthfa - where 22 per cent of pupils are entitled to free school meals - is a "very successful" school.
The inspectors' report praised the "clear vision and very effective leadership" of the head. Throughout the school, all teaching was found to be good or better, and was described as "a major strength of the school".
Standards of pupil achievement were also good or very good.
The report said: "Cyfarthfa high school ... is very successful in terms of its ethos, standards of achievement and quality of teaching."