An English teacher who repeatedly had sex with a teenage student in her car threatened to fail him when he tried to end their affair, police have said.

Danielle Watkins, who is married with two young children, repeatedly had sex in her car with the 18-year-old student during school hours, Stamford Police claim.

And when he tried to break-off the eight-month affair Watkins threatened to fail him, cops said.

The 32-year-old teacher, who taught English at Stamford High School, also sent the student nude photos and supplied him and a 15-year-old friend with marijuana, it is claimed.

Watkins, who has been a teacher for nine years, handed herself into authorities in Stamford on Thursday and has been charged with two counts of sexual assault, among other charges.

The alleged affair was revealed when the teen told a school counselor, who alerted the Department of Children and Families and the Board of Education.

The boy waited until he had finished classes before telling the authorities as, when he previously tried to end the affair, Watkins said she would fail him and accused him of seeing someone else, it is claimed.

The pair allegedly had sex numerous times in her car during school hours. It was parked off the school grounds, local newspaper The Courant claimed.

Watkins also allegedly let the student drive her car even though she knew he didn't have a licence.

On June 27, police officers pulled over Watkins's car. Inside they found marijuana and a cell phone, which is said to have contained evidence she had sent naked photos of herself to the boy.

On Thursday she was charged with sexual assault and sale of marijuana, and risk of injury to a minor in relation to a younger boy, says the NY Daily News.

Watkins has been placed on administrative leave pending the outcome of the investigation.

She was released after posting $100,000 bail and is scheduled to appear in court on July 31.