The start of term at Barcroft Primary School in Willenhall was delayed last week when 28 members of staff refused to work.
They called in sick on Monday and Tuesday in what is believed to be a protest at the reinstatement of two senior members of staff, who had been suspended over allegations of bullying and safeguarding breaches.
The school reopened to some year groups on Wednesday, with remote classes laid on for other pupils.
Now union chiefs say attempts to resolve the dispute had failed, meaning they have been "left with no alternative" but to ballot members at the school over strike action.
Paul Nesbitt, a national executive member for NASUWT in the West Midlands, said Walsall Council had been asked to confirm that all investigations and grievances against the two senior members of staff had been completed.
He said teaching unions had received no response and that industrial action "may be the only way forward".
Mr Nesbitt said: "They have had the opportunity to respond with a view to sorting out this very serious situation, but have not done so.
"As a result our members will be balloted over strike action due to adverse management techniques, bullying, safeguarding and health and safety concerns.
"Members are extremely concerned with what is going on at the school.
"We want the school open, but members rightly feel they have to protect themselves and the children.
"There is still time to sort this out before we take action, but the authority has to listen to the issues being raised by staff."
Staff are understood to be fully supportive of acting head Paul Drew, who took over in May this year after the school announced that headteacher Jasbir Johl and deputy head Marie Poole would not be in school "for the forseeable future".
A spokesperson for Walsall Council said: "We continue to work with the unions and the head and deputy headteacher to reach a solution to resolve the issues at Barcroft Primary School."
Meanwhile it has emerged that council chiefs are probing allegations of bullying at another primary school in the borough.
The Express & Star has been told that staff and governors at Salisbury Primary School in Wednesbury have been questioned regarding allegations of bullying against a senior staff member.