DALLAS (MEREDITH) – A Texas student allegedly blackmailed his former middle school teacher for nearly $28,000 by threatening to tell police about their sexual relationship, according to court records.

Thao “Sandy” Doan, 27, taught math at Raul Quintanilla Senior Middle School. She was arrested last Friday and faces a sexual assault charge in connection to the alleged relationship, the Dallas Morning News reports.

The teenager’s mother said she found suspicious text messages from the teacher on her son’s phone. Court documents state the mom reported their conversations to school officials on January 10. She also tipped off her son for allegedly blackmailing the teacher.

The school then handed those messages over to Dallas police. Officers reportedly arrested Doan on Friday and released her that same day.

Doan is facing a felony charge of improper relationship between an educator and student. The Dallas Independent School District said they have placed her on administrative leave, according to KDFW-TV.