The alleged victim was at south-east London's Bexleyheath Academy and was 15 when the affair is said to have begun.
Stuart Kerner, 44, denies two counts of sexual activity with a child and six counts of sexual activity with a child by a person in a position of trust.
The affair is said to have happened between March 2011 and October 2012.
The jury watched a video interview with the teenage complainant in which she told how her schoolgirl crush grew into something more serious.
"I felt special. But, I dunno, it wasn't really," she told a detective in the interview. "And admitting that does kind of hurt."
She said Mr Kerner told her that their relationship was "written in the stars".
The girl said when he kissed her for the first time: "I couldn't actually believe what had just happened. I was sort of in a little dream world, it was unbelievable."
She also said she could say "hand on my heart" that he would never have made a move on her without her encouragement.
In the video, the alleged victim told how she lost her virginity to Mr Kerner lying on a yoga mat on the floor of an empty room, having previously engaged in intimate acts on school grounds.
"I knew it was wrong, but it was something that felt right as well ... There was always a little voice at the back of my head saying, 'he's just using you'," she said.
The detective asked her: "Did you love him?"
"I thought I did," she replied.
The court has heard the girl had intimate knowledge of Kerner's life - including details about the layout of his home, his birthday and personal information about his wife - though the accused is said to have told police he had never swapped personal details with her.
"The Crown state that it's inconceivable that all these details could be the product of a teenage fantasy," prosecutor Warwick Tatford told the court.
The girl is expected to be cross-examined later.
A school teacher who allegedly pursued a secret sex affair with a student told her it was meant to be, Inner London Crown Court has heard.