A mother of three texted her 15-year-old lover: “I can’t get you out of my head”, a jury has heard.
A classmate of the youth said she read the message after he confided to her he was sleeping with Caroline Lea, who was more than twice his age.
Preston Crown Court was told the girl secretly recorded his confession and then showed it to school bosses who passed the matter to police.
The prosecution say Lea, 36, from Clifton Drive, Blackpool, had sex with the boy on a regular basis for several months and on one occasion was said to have given him drugs to keep him awake for sex.
Lea faces separate allegations of taking part in a threesome with a second 15-year-old complainant and his young friend, while a third complainant said he was sexually touched by her when he too was aged 15.
Giving evidence, a friend of the first complainant said he told her at school he had been having sex with the defendant.
She said to him “don’t be stupid” but was then shown the text message said to have been sent the same day by Lea.
Francis McEntee, prosecuting, asked her: “When you had seen that text did (the complainant) say anything?”
The witness said: “He just told me that he was starting to get feelings for her.”
She said she recorded the conversation on her phone without the complainant’s knowledge and handed it to the school authorities.
The witness confirmed to Rosalind Emsley-Smith, defending, that she saw the text message in the name of “Caroline”.
The barrister said: “You cannot be certain though that she was ultimately responsible for sending it?”
“No,” replied the witness.
The prosecution say the alleged sexual offences took place against a backdrop of Lea allowing young boys to smoke cannabis and drink alcohol at her former address in the town.
One of the complainants told police his friend suggested “a three-way” with the defendant and then it “just happened”.
Lea denies seven counts of engaging in sexual activity with a child on various dates between December 2013 and March 2014.
She has also pleaded not guilty to one count of allowing her home to be used for smoking cannabis and a separate count of supplying amphetamine, a Class B drug.