Hazel Boutel believes there is nothing she could have done to prevent an eight-year-old brandishing a knife in her school's playground last month.
The head of Pinewood primary in Romford, Essex, said: "You can't stop these things coming into school. What are you going to do, search children?"
She does not believe airport-type equipment is the answer. "How long is it going to take to get pupils into school? And would it show up a small key-ring penknife?"
Pinewood hit the headlines last weekend after a boy was suspended following an incident with a key-ring knife.
Mrs Boutel said: "He was showing off. But he did not hold the knife to anyone's throat as the papers said. We have not had anything like this happen before but people keep telling me, there but for the grace of God goes any headteacher."
Jenny Cooper, a teacher at a special school in Brent, believes schools need to do more protect staff. She told the National Union of Teachers conference last year how a 14-year-old with mental health problems came to school with a kitchen knife and attacked a support staff member.