He wrote about these years in his autobiography, borstal boy.
British Dictionary definitions for borstal
(formerly in Britain) an informal name for an establishment in which offenders aged 15 to 21 could be detained for corrective training. Since the Criminal Justice Act 1982, they have been replaced by youth custody centres (now known as young offender institutions)
(formerly) a similar establishment in Australia and New Zealand
C20: named after Borstal, village in Kent where the first institution was founded