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WEST DALLAS (CBSDFW.COM) – A Dallas ISD middle school teacher was charged with sexual assault for allegedly having an affair with a teenage student, who later went on to extort nearly $30,000 in exchange for his silence, according to police.

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Thao Doan, a 27-year-old teacher at Raul Quintanilla Sr. Middle School in Dallas, was arrested on Friday in connection to the suspected sexual relationship with the 14-year-old.

The student’s mother turned Doan in to the principal after she found inappropriate text messages a few weeks ago. She said the inappropriate teacher/student relationship started a year ago.

In addition to the texts, the teen’s mother was tipped off by the fact that her son had a lot of cash all of a sudden.

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“When I gave his little brother back the phone he received a message from the teacher that morning saying ‘I’ll be at a meeting until 11 o’clock and I’ll drop off the money to the address you told me too,’ ” said the student’s mother.

But she said she assumed he was selling drugs, not having sex and extorting money from Doan.

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