The pupil at Saltley School was originally expelled after an incident, a decision overturned by governors.
NASUWT union members voted to stop teaching the pupil last month and now GMB union members are to be balloted.
It comes on the day police said they investigated allegedly racist comments about the school on social media.
Police attended an incident involving a pupil with a knife on 22 May, but said no action was taken.
Head teacher Balwant Bains has previously said the "serious incident" action was dealt with by "due process" and that he had confidence in the "safety of staff and pupils".
The school has refused to comment further.
The specialist science college has about 950 pupils, aged 11 to 16.
Police have confirmed Mr Bains reported concerns that social networking sites were being used to claim that the "school was racist".
Ch Supt Alex Murray said: "Three people from the Alum Rock area of Birmingham received police warnings for their role in the messaging.
"Local officers continue to work closely with the school and the council."
More staff at a Birmingham school could join colleagues refusing to teach a pupil accused of threatening another one with a knife.