The girl, who cannot be named, told the court how Darren Caudle-Wood "wanted sex" with her while she was in the sixth-form at Pate's Grammar School.
The jury heard they had secret meetings during their eight-month relationship.
Mr Caudle-Wood, 37, of Church Road, Cheltenham, denies seven charges of having sex with a child under 18.
The girl told Gloucester Crown Court: "He used to say how much he loved me. I sat there and didn't say anything because I was very, very scared. As the term progressed I cared about him more and more."
'Candles laid out'
The girl went on to describe how Mr Caudle-Wood arranged a meeting at a friend's flat in the town which had been "prepared".
"There was a double bed and there were towels and sheets laid out. And there were candles laid out everywhere," she said.
"I was so scared. I thought 'Oh my goodness', I thought 'I don't know what he thinks but there's no way I'm having sex."'
The couple's alleged eight month affair became public after the teenager was summoned to see the head teacher and explain why she had been harassing Mr Caudle-Wood.
She then confessed they had been having an affair.
Mr Caudle-Wood was arrested in October 2005 but claimed the girl was a "fantasist" and that he had never been anything less than professional in his dealings with her.
The trial continues.
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