Headteacher's son James Davies, 23, confiscated the phone of a 15-year-old girl after catching her using in lessons.
Davies went into her mobile to get her personal number - and began sending her sexual text messages after class. Prosecutor Hywel Hughes said: "He rang her while she was in the bath and asked her to meet him, telling her to wear something sexy.
"She dressed, putting clothes on top of her pyjamas and met him in his car.
"They sat next to each other and he texted what he wanted to do to her.
Cardiff Crown Court heard their meeting at a country park was ended when a warden arrived to say he was closing up for the night, But after dropping her at home, Davies rang her later and said: "Go to bed and send me some dirty pictures."
The court heard their relationship became a source of gossip at the comprehensive school where Davies was working as a teaching assistant but regarded by the pupils as a supply teacher.
The woman deputy head overheard girls talking and police were brought in.
Mr Hughes said: "It turned out the girl was not the only pupil who had received this attention from Davies.
"He was texting another 15-year-old girl with messages including: "Would you come if I gave you detention?" and "Would you do things with me in the closet?" Another message asked if she would have sex with him and complimented her on her backside when she was wearing tight trousers.
Father-of-one Davies admitted four charges involving the two girls.
He pleaded guilty to two charges of sexual grooming, sexual activity with a child by a person in a position of trust and inciting a child to engage in sexual activity by a person in a position of trust.
Andrew Davies, defending, said he was "a young man of character, ability and potential whose career was now in ruins."
He said: "He has apologised to his victims and his family. It was out of character." Judge David Wynn Morgan said: "You targeted each of these children and made efforts to seduce and corrupt them.
"This was a gross breach of trust. You used mobile phone calls, text messages and internet social networking sites to groom these girls.
"You exploited your position by obtaining the mobile phone of one of the girls after getting her number and arranged to meet her to make sexual suggestions.
"You sent 10 to 20 text messages to the other girl. These children were frightened, confused and embarrassed by the situation."
Davies, of Laleston, Bridgend, was jailed for 12 months and banned from teaching for an indefinite period. Davies had studied to become a teacher at Trinity College in Carmarthen to follow in the footsteps of his mother - a headteacher at a primary school.
He worked at three schools in Bridgend, South Wales, as a supply teacher after leaving university. The schools cannot be identified. (Last Updated: 7:27PM GMT 26 Feb 2009 Telegraph)