Police said Sarah Fowlkes, a science teacher at Lockhart High School, was "engaged in sexual contact" with a 17-year-old at the school.
District officials say they were told of the report on Friday, March 10, and police immediately began an investigation.
According to a district press release, police determined that a male student had been in contact with Fowlkes in person and by message.
A warrant was issued for Fowlkes' arrest on Monday, and authorities said she turned herself in.
Fowlkes had been with the Lockhart Independent School District since Oct. 2014.