ST. LOUIS (AP) — A substitute teacher who tested a stun gun during class at a suburban St. Louis grade school will not face criminal investigation, a police official said Thursday.

The incident happened Wednesday at Kehrs Mill Elementary School in the St. Louis County town of Chesterfield. Principal Christina Garland wrote to parents that a substitute teacher brought a Taser — a high-voltage stun gun commonly used by police — into a fifth-grade class.

In a message to parents on Thursday, Garland wrote that the substitute teacher "tested" the Taser in the classroom but away from students. Garland did not explain how the weapon was tested, and a Rockwood School District spokeswoman did not immediately respond to a message seeking comment.

A teacher across the hall learned of the incident and contacted administrators. School officials immediately removed the substitute teacher, who is a woman. Another substitute was brought in for the remainder of the day. No one was hurt.

"Please be assured that at no time was there a threat to the safety of any student or staff member," Garland wrote. In fact, the students were involved in activities "and most may have been unaware of the incident at the time since it was handled in a quiet and efficient manner," she wrote.

The name of the substitute teacher was not released. Garland said in the letter that the school district is investigating and the woman is banned from substitute teaching at the school.

Police are not pursuing it as a criminal matter, according to Chesterfield Capt. Steven Lewis said, who said the teacher was carrying the Taser for personal protection.

"My understanding was it was just poor judgment on her part," Lewis said. "I believe she understood that."