WATERTOWN, S.D.(Watertown Public Opinion)- Calvin Hillesland, a Watertown High School teacher who gave four students a letter refuting their gender identities, has resigned.

Jeff Danielsen, superintendent of the Watertown School District, confirmed Hillesland’s resignation at Monday night’s school board meeting, but offered no other details.

The letters were given to students on April 25 at the school, something school district officials have acknowledged. The incident prompted an investigation.

Danielson said Tuesday morning that there is no update on the investigation.

In the letter, Hillesland tried to refute students’ gender identities and told the students that when they asked him to call their friends by masculine names, he felt it was “wrong” and “a lie.”

He also wrote in his letter that “biologically, every cell in your body is female, feminine. That’s the biological truth. The same is true for your friends.”

The letters prompted a small protest at the high school on April 26. At least one mother has pulled her student from the school district since the incident.

The school district issued a statement after the letters were handed out.

“The Watertown School District does not support this sort of action, and we respect the rights of our students to be who they are,” the district’s statement read. “We want to provide a safe learning environment for all students. We continue to work through the situation and ask for your support as we handle it.”