A Royal Marine reservist has been jailed for 12 months for having sex with a 17-year-old pupil while working as a teacher in South Devon.
Cpl Colin Kirkman,42, had an 'outstanding reputation' within 42 Commando, Exeter Crown Court was told.
But he returned from a bloody tour of duty in Afghanistan a 'cold' and changed man, according to a friend.
The court was told he met the girl while teaching in Torbay and the pair had sex at a hotel in Plymouth after she had been drinking one night.
They carried on having sex until after her 18th birthday.
Kirkman admitted three counts of sexual activity with a child while he was in a position of trust.
The offences came to light after the girl's friend, who had been in the hotel room on the night of the first sexual encounter, told teachers.
Warrant Officer Philip Gilby, who was Cpl Kirkman's regimental sergeant major in 42 Commando, described him as having an 'absolutely outstanding reputation' in his unit and at the training centre in Lympstone.
He was voted the 'most inspirational corporal' by recruits.
"I would have this man in my unit whether operationally deployed or commanding at the training centre," he said.
But the court was told that after his return from Helmand Province in 2011 Kirkman had changed. There had been 96 casualties and seven deaths in action during that deployment.
He suffered from post traumatic stress disorder, involving flashbacks and nightmares, his counsel said.
His partner said he had returned a 'cold and hard man'.
Judge Erik Salomonsen said: "The fact is you were in a position of trust and she was a child in your care.
"These are serious and significant matters and I regret to say in my judgement, and in respect of my public duty, for those in a position of trust undertaking sexual activity with children an immediate custodial sentence is inevitable."
The court was told he will be dishonourably discharged from the armed forces.
