A teacher has been suspended following claims tape was put over the mouths of pupils.
The incident is alleged to have taken place at Danesfield School in Williton, Somerset, yesterday.
Parents say the teacher applied the tape - which made one girl's lips bleed when it was removed - as pupils were being too noisy.
Police are aware of the claims and are "closely" liaising with the local authority, Somerset County Council.
Ian Bradbury, executive head teacher of the Quantock Foundation, of which Danesfield School is part, said a "thorough" investigation was taking place.
"We take all allegations concerning the well-being of our children very seriously," Mr Bradbury said.
"An incident happened yesterday which we are investigating thoroughly. A member of staff has been suspended whilst the investigation continues.
"My thanks to the numerous parents who have brought this matter to my personal attention and have given their on-going support to us in this challenging situation."
Parent Stacey Bradbury told the BBC her 10-year-old son had been left "really, really upset" by the incident.
"I can't put it in to words," she said. "Disgusted, shocked, traumatised by it all to be honest, to know that my son had to sit there quietly and have Sellotape put over his mouth.
"He was really upset, really upset. Last night he to be honest wasn't very keen on going to school this morning, which is really unusual for him.
"He didn't want to go and face obviously that he might have to see this teacher again."
Another parent, Rob Lovell, said his 10-year-old daughter had been "shocked" by what happened.
"When she removed it at the end of the lesson it took the skin off her lips and made her lips bleed," Mr Lovell told the BBC.
"She was a bit upset telling us because she thought she'd done something wrong."
Mr Lovell said his daughter told him most of the class had tape put on their mouths apart from a few who were allergic.
"When the Sellotape started to come off she put more on. She got them to keep it on for the whole lesson," Mr Lovell added.
A spokesman for Avon and Somerset Police confirmed the force was aware of the incident.
"We are aware of the incident and will be liaising closely with the local authority," the force spokesman said.