Feltham prison holds up to 800 inmates aged between 15 and 21, many with learning difficulties.
A report by the chief inspector of prisons in 2000 found "poor" and "unsatisfactory" provision, particularly in basic skills, but a later inspection in 2001 found "considerable progress" had been made.
This week the prison's deputy governor, Paul McDowell, said another inspection in recent weeks showed "further improvement".
Members of the Department for Education and Skills, the Adult Learning Inspectorate and the chief inspector of prisons inspect between 10 and 12 prison education departments every year and present a report to various education bodies to help gauge progress. The report is not published.
Mr McDowell revealed the inspection had taken place when he gave evidence at an employment tribunal in Croydon, south London, in a case brought by a former member of Feltham's teaching staff arising out of the 2000 report.