A parent told the Country Gazette the incident occurred Monday at the Danesfield School.
"My child said they were playing a game in a classroom and giggling. The teacher told them to be quiet, and when they didn't quiet down they put tape over all their mouths," the unidentified parent said.
Concerned parents visited the head teacher, Ian Bradbury, Tuesday to voice their complaints, the British news website thisiswestcountry.co.uk reported.
"We are investigating thoroughly. A member of staff has been suspended," Bradbury said.
Avon and Somerset Police said there were aware of the incident.
The teacher was unidentified, and the ages of the children allegedly involved were not immediately available.