Two boys were taken to hospital after receiving neck wounds during an opening night performance of Sweeney Todd at a school in Auckland, New Zealand.
The musical features a barber who murders his customers by slitting their throats and selling their remains to a pie shop.
One of the boys, both aged 16, was more seriously hurt but Auckland Hospital said both were in a stable condition.
Saint Kentigern College said it was "distressed" over what happened.
The head of the private college, Steve Cole told TVNZ, that it was an "unfortunate and isolated incident" involving "a prop that was covered in all sorts of duct tape and silver paper."
The razor did not have a sharp edge and it had been used numerous times in rehearsals, he said.
The school has postponed Thursday's performance of the play, and said the boys were expected to be discharged from hospital on Thursday
"Plays and performances are a big part of all school life" it said in an earlier statement.
"We are also very focused on helping other cast and crew members, along with the wider school community and have offered all students counselling."
Police are also investigating the incident, but said they were treating it as an accident.
"It's not a criminal inquiry. In this instance the play is not the reality," said the police spokesman, according to local media.
New Zealand's health and safety authorities are also reported to be investigating.