The school says it does "its utmost to ensure
the safety of all pupils and staff"
Staff have voted to refuse to teach a pupil who has been allowed to return to a Birmingham school after threatening other children with a knife.
Saltley School originally permanently excluded the teenager, but the decision was overturned by its governors.
Head teacher Balwant Bains confirmed a "very serious" incident had been dealt with "through due process".
Police said they took "no further action" after an incident involving a pupil and a knife on 22 May.
A parent at the school, who did not want to be identified, told the BBC she was concerned for the safety of pupils.
'Really scares us'
"This young boy threatened several children and you can imagine the dilemma we're facing.
"Is this boy going to hurt one of our children [and] what happens if he does hurt one of them? It really does scare us," she said.
Anne Brimecombe, from the teaching union NASUWT, said this kind of ballot was "very rare" and was only taken when "members have an issue with one particular pupil".
"This is not like a normal strike - there are sensitivities around other individuals and children which is why we cannot disclose any more details," she added.
The specialist science college has about 950 pupils, aged 11 to 16.
Mr Bains, who is also governor at the school, said: "We did have one incident recently that I considered to be very serious and it went through due process.
"The needs of all pupils are being met."
The added that the school did "its utmost to ensure the safety of all pupils and staff".