A kindergarten teacher who gave melatonin gummies to special education students at an elementary school in Humble, Texas, has resigned, school officials said.

The Pine Forest Elementary School teacher gave the gummies "to a few of her students," and the district learned of it on September 28, Humble Independent School District spokesperson Jamie Mount told CNN in an email.

Melatonin is a widely available over-the-counter sleeping aid that's considered a dietary supplement and is not regulated as a drug by the Food and Drug Administration.

A school district investigation found that the teacher "did give out melatonin, acting on her own and without obtaining parent permission. The teacher also did not notify campus administration nor the nurse," according to Mount.

"The teacher's actions were unacceptable," Mount said. "We are appalled that the teacher made this decision."

The students were doing well, and the parents of all students in the class have been notified by the principal, Mount said. The district also reported the incident to police, the State Board for Educator Certification and Child Protective Services.

Asked about the teacher's resignation, Mount said, it is "the fastest, most efficient process to separate employment."

"Terminating a teacher's contract can be a longer, drawn-out process because the state education code requires a multistep process to be followed." Mount said. "With the Resignation, the matter is still reported to the State Board for Educator Certification."

CNN has reached out to the Humble Independent School District Police and the Texas Education Agency's State Board for Educator Certification for information.

Humble is a suburb of Houston.

The use of melatonin appears "to be safe for most children for short-term use," according to the National Center for Complementary and Integrative Health, a department of the National Institutes of Health.

But the agency also points out that "there are uncertainties about what dose to use and when to give it, the effects of melatonin use over long periods of time, and whether melatonin's benefits outweigh its possible risks."