The male teacher taped up the girl's mouth as she readied to serve lunch to others in her class in Tochigi, north of Tokyo, because she had forgotten to bring her own face mask, the Sankei Shimbun reported.
Japanese schoolchildren serve and eat their school lunches in their own classrooms. Most schools require them to cover their mouth and nose while they serve.
The teacher apologised to the girl and her parents, the paper said, after an anonymous call was made to school authorities.