Michael Day, 47, was teaching at Morriston comprehensive school in Swansea when he lured the drunken boy into his bed.
He had already handed in his notice by the time the school became aware of the allegations against him, and never returned from sick leave.
Cardiff crown court heard Day's career was in ruins and he would not teach again.
Day, who has a black belt in judo, was coaching the 15-year-old at a club in the city. But the court was told last week that Day and the boy went on a drinking session together.
He later put the boy to sleep in his own bed and got in himself "to make sure he didn't drown in his own vomit".
Judge William Gaskell told him: "You claim you both became aroused when lying in bed together - but you are a much older man who was in a position of trust.
"The boy was said by his mother to have been in a terrible state when he woke up and realised what had happened.
"His impression was that the sexual activity went considerably further than you were prepared to admit. I accept you are not a sexual predator - but you have lost everything you worked so hard to build up."
Day, of Bonvilston Road, Pontypridd, was jailed for 12 months after admitting sexual activity with a child while in a position of trust.
A spokesperson for Swansea council said all the appropriate child protection checks had been made on Day before he was employed at Morriston on a temporary contract.
He added: "Michael Day had given notice to resign before the school became aware of the allegations against him. The teacher went on sick leave and he never returned to the school.
"No allegations were ever made to the headteacher of Morriston comprehensive or the local education authority about this teacher in relation to the school or its pupils.
"The school has a coherent child protection training programme and a designated member of staff dealing with child protection who is appropriately trained. Units on child protection are included in the personal and social education curriculum and information on ChildLine is included in pupils' planners."