Eleanor Wilson, 29, is accused of fondling the boy as she sat next to him on the flight before beckoning him into the toilet and performing a sex act on him.
They then had full sex in the cubicle before going back to their seats.
Bristol Crown Court heard that Wilson later told the boy she was pregnant and that he was the father, but that she would have an abortion.
The defendant and the boy, who had both been drinking during the flight, had been part of a group of staff and pupils on a school trip during the summer holidays of 2015.
Prosecutor Virginia Cornwall said that during the trip Wilson and the boy, who cannot be identified for legal reasons, spent a lot of time together.
“It was the circumstances in which the two interacted on that trip that led to an erosion of boundaries and it was her responsibility, the Crown say, as the adult, to ensure there was no erosion of boundaries,” she said.
“She was a 26-year-old woman and he was a teenage boy.
“It was her choice to be with him. There were a lot of nighttime chats where others had gone off to bed.
“He was being treated on a par, an equal basis, as if an adult.”
Miss Cornwall said that Wilson and the boy had been drinking on the flight home.
“It was the alcohol, you might find on the evidence, contributed to the lack of inhibition on the part of the defendant,” the prosecutor said.
“Such intimacies and attraction built up during the days and nights of the trip, which resulted in close contact on the plane.”
She beckoned him into the toilet, where they had sex, Miss Cornwall said.
“Having had sex they went back to their seats.
“It was a secret between them.”
After returning home, their “clandestine” relationship continued, with Wilson and the teenager going for a meal in Nando’s with others from the trip and also having days out alone together.
The court heard that the boy bought Wilson a bunch of flowers and chocolates, she gave him a lift home and he showed her his bedroom where they kissed.
They also allegedly had days out together at Tintern Abbey and Ashton Court, going for pub meals and drinking pints of cider together.
The jury was told they had swapped mobile phone numbers and Wilson rang him while she was on holiday in Italy with her boyfriend.
The boy’s father noticed his son’s behaviour had changed and he talked “incessantly” about the defendant and said he was going to split up with his girlfriend.
The teenager had told friends about the relationship but sworn them to secrecy, and when the pair returned to the school for the start of the new academic year, rumours circulated.
Wilson was spoken to by the school’s head and deputy, but denied it and “appeared shocked”, Miss Cornwall said.
She later allegedly told the boy she was pregnant by him.
“She told him because she was scared. She decided to have an abortion and she kept it from her boyfriend, who she says was the father,” the prosecutor said.
“There is no doubt there was a pregnancy but she does not accept that she had intercourse with the boy or that he was the father.
“She says she confided in him because she had no-one else.”
Wilson, of The Rope Walk, Dursley, Gloucestershire, denies four charges of sexual activity with a child by a person in a position of trust.
The trial continues.