Born in Jamaica, Sir William moved to Britain at the age of seven. His first teaching appointment was in Portsmouth and he came to Phoenix High School in Shepherds Bush as Headteacher in 1995, having previously held headships at Cranford Community School, Hounslow, and Copland Community School, Brent.
In September 2010 he became Executive Headteacher of The Phoenix Canberra Schools Federation. His excellent leadership at Phoenix, overcoming low results and bad behaviour, was reported to have been the inspiration for the head teacher character in the BBC's school drama, Hope and Glory.
In 2008, Ofsted inspectors praised Sir William's role in the school's continuing improvement: 'The school is exceptionally well led by a charismatic, indefatigable head teacher who receives excellent support and challenge from a fully committed governing body,' said the inspectors.
Sir William was a member of the DfEE Standards Task Force between 1997 and 2001. He also sat on the Home Office Justice Task Force and the Home Office London Youth Crime Reduction Task Force. Until 2006 Sir William was a member of the London Black and Minority Ethnic Cracking Crime Partnership Board and until December 2009 was Deputy Chair of the UK Judging Panel for The Teaching Awards. He was also a member of the British Board of Film Classification advisory panel on children's viewing until the summer of 2010.
He is currently a trustee for Research Autism, a commissioner for The Family Commission, a national inquiry into the extended family and the support it needs in the 21st century and on the Board of Directors of the Lyric Theatre in Hammersmith.
In January 2010 the National College for Leadership of Schools and Children's Services selected Sir William as a National Leader of Education and designated Phoenix High School as a National Support School.
Sir William has also been widely featured in the media, including the Channel 4 documentary series 'The Unteachables'. He has also contributed to a range of television and radio programmes, including Newsnight, Panorama, BBC Question Time, Breakfast Television, The One Show and Inside Out and Radio 4's The House I Grew Up In and Saturday Live.
Sir William was made a knight in June 2008 in the Queen's birthday honours, for his services to education and community.