John Coatman, 76, was found guilty in 2016 of gross indecency with a member of a youth group he was involved with in the 1970s.
His conviction was thrown out by the Court of Appeal last April because of an error on the charge sheet put before the jury.
When the case returned to the Old Bailey for a retrial last month, Coatman, of Croydon, south London, pleaded guilty to an amended charge of indecent assault.
Yesterday he was given a 21-month sentence, suspended for two years, and a 12-month supervision order.
Judge Anne Molyneux told Coatman, who has inoperable cancer: “You were in a position of considerable trust.”
But she added: “At the time of the offence you were struggling to cope with the pressures of your life.”
The court heard that, after being arrested, Coatman confided in a friend that he had been wrestling with his sexuality.
The teenager would visit his home, where they would engage in “rough and tumble”.
Later the pair undressed in Coatman’s bedroom. Coatman was head of St Andrew’s secondary school in Croydon but his victim was not a pupil at the school, the court heard.
He retired in 1998 after 42 years as a teacher and was made an MBE in 2012.
Prosecutor Mark Trafford QC, said: “The defendant’s MBE for services to young people was something that the complainant found difficult to comprehend under the circumstances.”