Roland Wright, 83, who taught at Caldicott Boys' Preparatory School in Farnham Royal, Buckinghamshire, assaulted five pupils aged between eight and 13 between 1959 and 1970.
His sentencing at Amersham Crown Court went ahead despite the death of another former teacher who had been due to be sentenced with him. His former colleague, Hugh Henry, 82, apparently threw himself under a train on Tuesday.
Judge Johannah Cutts QC told Wright: "These offences were so serious that it is beyond question that only a custodial sentence can be justified.
"I have to sentence you for multiple offending against each of these victims.
"This was prolonged activity, the boys were groomed by a master at a school responsible for their care."
Mr Clegg, who was joint head boy at Caldicott in 1980, spoke of his horror after Wright's conviction at Amersham Crown Court in December for 10 counts of indecent assault on a male and two of indecency with a child.
He told a newspaper that he was "shocked and appalled by the gross betrayal and violation of childhood innocence shown to have taken place".
He said: "These reports will disturb everyone, but most especially those, like myself, who were pupils at the school and were entirely unaware that such abuse was taking place.
"I can only imagine the devastating impact that such abuse must have had on the young boys who were affected.
"My heart goes out to them."