A HIGH school teacher who had sex with three of her students has been jailed for two years.

Casey Lee Sullivan, 33, appeared before Chief Justice Alan Blow in the Supreme Court in Hobart today after earlier pleading guilty to nine counts of having sexual intercourse with a young person under the age of 17 years between 2012 and 2015.

Chief Justice Blow said Sullivan had sex with a boy she met while still a student teacher in 2012 because he had paid her attention.

The following year, Sullivan sent flirtatious messages to a 15-year-old student via Facebook and later picked him up from his home and had sex with him in a car at a beach nearby.

In 2015 she contacted another student via social media and later took him into the school gym and had sex with him on the gym mats.

The teenage boy told his friends about the incident the next day and showed them a message from Sullivan to prove his claim.

He told a friend and a cousin about the incident, saying that another teacher had nearly walked in on them.

Sullivan had sex with the boy on further occasions in September and October and sent him sexually explicit text messages which included indecent images.

Chief Justice Blow said rumours of the encounter spread and police were called. Sullivan confessed to having had sex with all three boys.

He said the crimes had cost Sullivan her job and had exposed her and her family to abuse and ridicule.

But despite her regret for the offending, the judge said that jail was the only appropriate sentence in a case involving repeated sexual offending over a number of years.

He jailed Sullivan for two years, with eight months of the sentence suspended on the condition she not commit another offence for two years on her release.

Sullivan will be eligible for parole after eight months. Chief Justice Blow also placed her on the sex offenders register for seven years.