A married teacher accused of having an affair with one of her pupils asked the boy's girlfriend "intimate" questions, St Albans Crown Court has been told.
Jenine Saville-King denies having sex with a pupil
Jenine Saville-King, 29, from Hook in Hampshire, denies having sex with a child and abuse of trust.
It is alleged she was sleeping with the boy when he was aged 15 and 16, while she was a teacher in Watford, Herts.
The boy's girlfriend told the court Mrs Saville-King seemed "over-interested" in "physical details" about him.
The girl, who was 14 at the time of the alleged conversation, said: "Teachers don't ask about your relationships.
"She seemed to ask a lot of questions."
Prosecutors say Mrs Saville-King continued the affair with the boy after she became pregnant by her husband, Paul, 37, and gave birth to a son.
The girl, who is now 17, said there were rumours at the school about Mrs Saville-King's relationship with the boy and about the boy being the father of the child.
The girl said she logged on to an instant messenger website using the boy's identity and sent messages to Mrs Saville-King.
The girl said she pretended to be the boy and wrote "I love you".
She told the court that Mrs Saville-King had messaged back, writing: "I love you too, you know that don't you?".
Jurors were told that the boy, now 18, was a talented soccer player and had gone on to have a relationship with a 35-year-old woman after leaving school.
The affair is said to have taken place in 2004 and 2005. Neither the boy nor the girl can be identified for legal reasons.
The trial continues.