Scottish Labour leader Anas Sarwar has joined striking staff on the picket line of the private school both he and First Minister Humza Yousaf attended.
It is understood Mr Sarwar spoke to some of the staff at Hutchesons' Grammar in Glasgow, who were taking industrial action on Friday in a row over pensions.
The action comes after a vote by members of the EIS teaching union at the independent school earlier this month saw 78 per cent of them support the strike.
Mr Sarwar has previously written to the school's authorities to raise his concerns on the issue of pensions.
The EIS said school staff were sent letters over the Easter break saying they would have to sign new contracts, with the jobs of anyone who refused being advertised elsewhere. The strike on Friday is the first in the history of the school.
EIS general secretary Andrea Bradley said: "The fact that EIS members at the school backed strike action so convincingly, and have taken to the street today in protest at the school's actions, demonstrates the strength of feeling amongst teachers at this damaging enforced change to their terms and conditions of employment.
"The school's actions throughout this process have been shameful – from launching a consultation amongst staff which they then ignored the result of, to issuing 'fire-and-rehire' letters to teachers over a holiday period, to telling staff that their jobs would be advertised if they did not sign up to new inferior contracts within a very limited timescale – the management of the school have demonstrated that they do not value the views and contributions of Hutchesons' highly dedicated and highly professional teachers.”
The situation has also prompted strike action from teachers in the NASUWT trade union.
A Hutchesons' spokesman said: "The board of governors is extremely disappointed that the EIS union and some of its member teachers at Hutchesons' have taken strike action.
"The board also strongly rejects accusations of 'fire and rehire'. The school carried out a 60-day consultation, which is longer than legally required to ensure it was a thorough process and not rushed. It included both collective consultation and individual consultations.
"The board has been very clear that the reason for withdrawing from the Scottish Teacher Pension Scheme (STPS) is to cap pension costs at a sustainable level to protect the jobs of teachers going forward.”