Allison Marchese will be sentenced in March after striking a plea deal with authorities in her case

A high school teacher is facing jail after sending racy photos and text messages to two teenage students and performing oral sex on one of them in a classroom, it has been reported.

English teacher Allison Marchese, 39, sent flirtations and sexual hints through social media as well as a selfie of herself "dressed in only thong style underwear with her breast covered and her midriff exposed", court records said.

One message said that Marchese "would do anything (the pupil) told her to do as long as he was going to talk to her", an arrest warrant affidavit said.

The student at Daniel Hand High School in Madison, Connecticut, USA, was over the age of 16 but it was later determined another child had an inappropriate encounter with her as well.

Allison Marchese was a keen user of social media, sending messages and photos

The younger student was only 14 when their relationship began.

The arrest warrant said the teacher would ask the teen out for lunch and "became very forward and wrote things such as she could not concentrate during class because he was so good-looking."

The pupil had asked her to stop but she told him he had broken her heart and betrayed her.

The older pupil, aged 17, told police he received a provocative photo through Instagram and flirtatious texts on his cellphone.

In December 2014, the pair kissed and had oral sex in a locked classroom, the affidavit said, noting that the student later told her was freaked out by the incident.

The affidavit said he plotted to blackmail the teacher for $200.

Marchese had faced charges of second-degree sexual assault and risk of injury to a minor – sending her to prison for up to 20 years, The Hartford Courant reports.

But the website said a plea deal had been struck with prosecutors, allowing Marchese to plead guilty to lesser charges in exchange for the three-year prison term.

She will be sentenced on March 3.

The Canadian mother-of-two pleaded guilty to two counts of first-degree unlawful restraint, two counts of second-degree reckless endangerment, one count of second-degree harassment and one count of second-degree threatening.

It was done under the Alford doctrine, allowing her to not admit guilt to those charges but concede prosecutors had enough evidence for a trial conviction.

Marchese was ordered by Judge Melanie Cradle to undergo sex offender treatment.

She would also be on the Connecticut sex offender registry for 10 years.