St Kentigern College, Auckland head Steve Cole says two boys' necks were cut during the Wednesday opening performance of Sweeney Todd in an "unfortunate and isolated incident" with a prop razor wrapped in duct tape.
Mr Cole told TVNZ it was "a prop that was covered in all sorts of duct tape and silver paper, this had been done innumerable times during rehearsals, innumerable times even during the show.
"It was a razor that you would expect to see in a production of Sweeney Todd. But it didn't have a sharp edge. I'm very confident that the health and safety situation was strong," he said.
"It was an unfortunate isolated incident."
He said it was not unknown for schools to use props like this, adding the razor's edge had been filed down and bound.