Jonathan Thomson-Glover, 54, pleaded guilty to four counts of indecency with two children at a school and at a holiday home in Cornwall.
Thomson-Glover is already in jail at HMP Channings Wood for making indecent films of more than 120 pupils using hidden cameras at Clifton College.
At Truro Crown Court, Recorder Martin Meeke QC sentenced Thomson-Glover to an additional six months in prison.
The court heard Thomson-Glover "befriended and charmed" the parents of the boys so they trusted him to take their sons to a holiday home.
The abuse took place at some point between 1987 and 1992, probably when the boys were 12-years-old.
One of the victims confronted Thomson-Glover in 2001, but the teacher assured him he no longer engaged in any sexual behaviour with boys.
Recorder Meeke said Thomson-Glover had committed a "gross breach of trust".
He said Thomson-Glover's reputation and career had been destroyed and noted he had engaged with a sex offender's treatment programme at prison.
Thomson-Glover was given six-month sentences on each count to be served at the same time, but in addition to his current sentence.
The former teacher was originally arrested in August 2014.
Det Insp Andrea Kingdon, from Devon and Cornwall Police, said Thomson-Glover was a "manipulative and devious offender".
She said: "These crimes amounted to an abhorrent abuse of trust and we praise the courage of the victims in this case for coming forward."
A teacher sexually abused young boys at a school and at his holiday home.