A veteran teacher and mother-of-two from Texas is facing 21 charges after admitting to having sex with four high school students.

Heather Lee Robertson, 38, of Lufkin, was indicted on Thursday on six charges of improper relationship between educator and student as well as 15 sexual assault charges, county records show.

Robertson was taken into custody in late April and initially charged with four counts of improper relationship between educator and student. It is also alleged that she had a threesome with two of the boys.

Heather Lee Robertson (pictured in her mugshot, left), 38, is facing 21 charges for having sex with four high school students after sexting with them on Snapchat (pictured right)

Robertson was initially facing 20 years in prison for the underage relationships but the sexual assault charges may have added 20 more years to her prison sentence.

The Texas also had a court hearing on May 1 for an unrelated DWI charge from March; she pleaded not guilty.

According to an arrest warrant, the investigation into the Hudson Independent School District teacher got under way on April 20 when police got a tip about Robertson’s alleged involvement with several of her students.

One teenage boy told detectives that he and Robertson, formerly a kindergarten teacher in the district, started talking and exchanging sexually explicit messages on Snapchat, and sometime after spring break, she invited him over for sex.

The teen asked whether he could bring along a friend, and Roberson agreed, according to the document cited by the Lufkin Daily News.

When the two juveniles arrived at the woman's apartment, they found her lying on the couch and smoking an e-Cigarette.

After a few minutes of conversation, Robertson invited her underage guests into the bedroom, where she proceeded to strip naked before having a threesome with the high schoolers, according to the arrest document.

Two other high school students later admitted to meeting Robertson for sex on multiple occasions, bringing the total number of victims to four.

One of the boys recounted for the police how he would sneak out of his home and Robertson would pick him up and take him to her apartment to have sex.

Robertson (pictured with her eldest daughter) admitted to the allegations last month, confessing that she had sex with one of the students four times between late 2016 and April 14

When investigators confronted the 38-year-old educator, who has two daughters of her own, Robertson admitted to the allegations, confessing that she had sex with one of the students four times between late 2016 and April 14.

Robertson said she recently became a heavy drinker and would sometimes forget the details of the sexual trysts. She also revealed that she did not force the teens to wear condoms because she can no longer get pregnant.

The 38-year-old said she instructed her underage lovers to keep quiet about their sex sessions, lest she get in trouble.

Hudson ISD Superintendent Mary Ann Whiteker said in a statement that Robertson resigned just after the allegations against her came to light.

She had been with the district since 2000, spending the first 13 years of her teaching career at Burley Primary in Lufkin.

Robertson was pulled over by police while driving in Lufkin at around noon on Saturday and was arrested on the improper relationship charges.

The teacher's past criminal record includes the single arrest of driving while intoxicated count last month.

Robertson is currently in Angelina County Jail on a $200,000 bond.