Richard Alston, 70, of Vinery Road, Bury St Edmunds, Suffolk, worked at Cavendish School in Ealing, west London, between 1975 and 1980.
He was the partner of the founder of the Paedophile Information Exchange.
Alston was sentenced at Southwark Crown Court for gross indecency with a child under 14 and indecent assault.
The court heard Alston and his partner Peter Righton forced an 11-year-old boy at the school to watch pornography and then perform sex acts on him at their home in Greenford, west London.
Righton, who died in 2007, was a founding member of the group set up in the 1970s that campaigned to lower the age of consent.
Righton was 19 years older than Alston, who was 16 when they met. They spent 40 years together, the court heard.
It was the investigation into Righton - convicted of importing images of child abuse in 1992 - that led to MP Tom Watson using Parliamentary privilege in 2012 to allege there was "clear intelligence" of a VIP child sex abuse ring.
The victim, whose anonymity is protected, came forward to police in 2013.
The abuse happened between February 1978 and February 1980 when the boy was at the school described at the time as being for "maladjusted boys".
Judge Alistair McCreath told Alston: "I acknowledge that at the time you committed these offences you were in a very close relationship with an older man who had very particular and aggressive views about the propriety of sexual behaviour with minors."
He said while he had the "intelligence, maturity and ability to say no" to "some extent at least" his behaviour was influenced by Righton.
The court heard Righton and Alston's friend Charles Napier - now a convicted paedophile - would also be present on some occasions.
Napier, a former teacher from Sherborne in Dorset, was at one time treasurer of the Paedophile Information Exchange.
Alston was found not guilty of four counts of indecent assault and two counts of indecency with a child.
The jury was unable to reach verdicts on one count each of the same offences and they will lie on file.
A former remedial school teacher has been sentenced to a year and nine months in prison for the sexual abuse of a pupil in the 1970s.