Teachers across London this week followed the lead of colleagues in North Finchley in voting not to cover for unfilled posts.
The industrial action, announced on Tuesday, raises the prospect of children being sent home from the borough's schools within days.
Secondary schools in Barnet alone are more than 50 teachers short but have been coping until now.
Earlier this year NASUWT union members at Christ Church in Warnham Road, North Finchley, became the first in the country to vote not to cover for unfilled vacancies.
The school was potentially within days of closure when five South African teachers were flown in to fill gaps.
This week the two largest unions, the NASUWT and NUT, balloted members in London and Doncaster saying they were no longer prepared to "paper over cracks" at schools unable to recruit staff.
Yesterday28/2 was the cut-off date for resignations for staff wishing to leave Barnet's schools after Easter, leading to fears the borough would see a staff exodus.
Alan Holmes, Barnet's NASUWT branch secretary, said: "That is going to affect quite a lot of schools. There is a normal ten per cent turnover of staff which will leave schools with posts to fill."
But Councillor Alison Moore, Barnet Council's education spokeswoman, was not unduly concerned. She said: "It [the ballot] illustrates that this isn't specifically a Barnet problem.
"Schools and the LEA have been working hard to meet the challenges of recruitment and retention and will continue to do so."
Council officers are this week working to organise a teaching recruitment fair to be held in the borough on March 12.
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