LUFKIN, Texas -- A 39-year-old East Texas kindergarten teacher has been sentenced to 10 years in prison after pleading guilty to seven counts of sexual assault with a child and improper relationship with a student for having sex with high school students.

A judge in Angelina County also has ordered Heather Robertson, of Lufkin, to surrender her teaching certification and register for life as a sex offender.

Robertson was a teacher in the Hudson school district, just outside Lufkin, for 13 years. Prosecutors said Thursday an investigation showed she had sex at her Lufkin apartment with at least six Hudson students between last November and February.

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After her arrest, school district police said in an affidavit Robertson admitted to having sex with four students, two at the same time and two others on multiple occasions.

According to the affidavit, a Hudson Independent School District police officer began investigating the case on April 20, after receiving information about inappropriate relationships between a teacher and several high school students.

One student told police he had been sexually involved with Robertson for the past few weeks. He said he started chatting and sexting with Robertson through Snapchat shortly after spring break and Robertson asked him to come to her apartment so they could have sex. He said he was with a friend, another student, and Robertson said it was OK for the friend to come along.

Both students told police they had sex with Robertson at the same time during the encounter.

The officer then spoke to two other students, who said they had sex with Robertson on a few occasions. They said they would sneak out of their houses and Robertson would pick them up and take them back to her apartment to have sex and then take them back home.

The officer then spoke to Robertson, who admitted to the allegations, according to the affidavit. She said she had recently become a heavy drinker and would sometimes not remember the details of the encounters.

She resigned as a Hudson teacher April 20 and was arrested the next day. She could have received up to 20 years on each count.