A teaching assistant texted naked pictures of himself to a schoolgirl while she was in class.
Samuel Bradley, 23, contacted the 16-year-old on Facebook and arranged secret meetings with her outside of the classroom.
He also groomed other girls while working at the school in Rotherham, South Yorkshire. His offences came to light when the mother of the girl contacted police.
Detectives found Bradley, from Dinnington, near Rotherham, had 40 indecent images of his victims in his possession.
He was sentenced to three years prison at Sheffield Crown Court after he admitted a total of eight offences against three girls; one of sexual activity with a child between 2011 and 2012, three of making indecent images, two of sexual activity with a child while in a position of trust, and two of causing a girl to engage in sexual activity.
Detective Inspector Mark Monteiro, from South Yorkshire Police, said: "The sexual contact took place off the school premises but some of the texting was going on under the desk in the classroom.
"He had worked at the school for some time and used his charisma to engage in inappropriate conduct with the children. This is a clear case of child sexual exploitation and a teaching assistant abusing his position of trust.
"I would urge anybody who has been a victim of similar behaviour to come forward and speak to our specially trained officers."
Bradley has been placed on the Sex Offenders Register for life and he has been barred from working with children in the future.