Robert McWilliam, 68, exchanged "sexualised" text messages with the girl when she was 16 and 17 years old at Berwickshire High School in Duns.
He was found guilty after a trial at Selkirk Sheriff Court of engaging in sexual activity with the teenager between June 2017 and June 2018 by touching her on the body and sending inappropriate messages of a sexual nature on a mobile phone he had supplied her.
Depute fiscal Joanna Waller said the crux of the case was that McWilliam was in a position of trust at Berwickshire High School.
But she read out a series of sexual messages which McWilliam of Ford Court, Gordon, had sent the girl.
The court heard that McWilliam was part of the Social Emotional and Behaviour Needs team at the school, but when the girl turned 16 years of age she was no longer under his care.
However, he also ran a drop-in centre at the school, which the girl sometimes attended, during which the messages were exchanged.
The divorced grandfather claimed as the girl was no longer under his care it was not an abuse of a position of trust.
But under cross-examination, Ms Waller asked McWilliam why he had conversations with the girl which were of a sexual nature which included she was making him 'hard”.
She referred to messages where he said he was excited about slapping her on the backside and urging her to delete other messages.
McWilliam responded he did not want the girl to feel "rejected" as she was described in court as a "troubled soul" and added he "wanted to make her feel good about herself”.
He added:"They were mostly replies, they were not instigated."
He said he had given her a mobile phone as he was getting a new one and was trying to help the teenager feel more secure.
The court heard that the girl sent McWilliam a picture of her breasts to his phone, in which he said “did put me in a position that I was concerned", but he did not report the matter.
Sheriff Peter Grant-Hutchison found McWilliam guilty of the charge of engaging in sexual activity with the teenager while in a position of trust.
Sentence was deferred until February 28 for the production of background reports and McWilliam was made subject to the notification requirements of the Sexual Offences Act.