The boy, who was 15 at the time, told the jury that teacher Renate Williams, 32, had "led him astray" during the trip, in November 1998.
Giving evidence through a video link, he said: "I wasn't exactly keen, I wasn't exactly gagging for it. I did not know what to expect. I had never done it before. She led me astray. She just kept nagging and I gave in. It was quite flattering at the time.
"Even if she was not the best-looking teacher in the world, it was still quite flattering."
Worcester Crown Court was told Miss Williams, of Poole, Dorset, acted in a sexually provocative manner by stripping and swimming naked in the sea in front of pupils and had made it "very plain" that she wanted to have sex with the boy.
The jury was told that Miss Williams, who had worked at the boys' residential school in Worcestershire for less than two months, later woke the pupil and took him to her room where the pair had sex.
But under cross examination the youngster admitted a second allegation of sexual intercourse between the pair at the school in December 1998 had not taken place, but said they had touched and caressed each other at Miss Williams' studio in the school grounds.
Andrew Fisher, defending, said the allegations against his client were false and that similar claims by pupils at the boarding school had resulted in another female member of staff leaving.
Miss Williams denies two counts of indecent assault. The trial was adjourned until today.Reuse content