Amy Gehring arriving at court on Tuesday
The pupil, who was 15 at the time of the alleged incident, claimed he was seduced by 25-year-old Amy Gehring on 31 December, 2000, while she was employed as a supply teacher at his school in Surrey.
Miss Gehring, a Canadian national living in Hampton, London, denies five counts of indecent assault against three boys, two aged 15 and one aged 14.
The boy, who was a virgin at the time of the alleged offence, told the court the pair had sex because they were both drunk and that Miss Gehring begged for forgiveness afterwards.
'Fumbling around'
The child, known as boy C and now aged 16, told Guildford Crown Court he was at the party with around 20 of his school friends.
He told the court: "I led her into the bathroom and we were chatting and fumbling around.
"We ended up having sex. We were both drunk, nothing would have happened if we weren't."
He added: "I was not annoyed with her, more annoyed with myself. I didn't want to lose my virginity that way."
He said the following day she phoned asking him to forgive her, declaring: "Oh my God, I am so sorry."
In a text message following their conversation she wrote: "Did we have sex last night? I can't remember, please don't think bad of me."
Questioned by Andrew Thompson, defending, the boy admitted not telling the truth to police in an interview.
He said he did not want his friends to go through a similar ordeal of being interviewed by officers and having to tell parents what had happened.
"I didn't lie. I twisted the truth to protect my friends," he said.
Earlier in the trial the court heard Gehring seduced two brothers and had sex with them in an alleyway near a public park.
The trial continues.