Another union representing teachers at the school once attended by First Minister Humza Yousaf will start the first of four planned strike action days.

Members of the NASUWT at Hutchesons’ Grammar in Glasgow will walk out on Tuesday over claims their pensions have been downgraded and they have been asked to sign “inferior” contracts.

Scottish Labour leader Anas Sarwar also attended the independent school, which parents can pay up to £16,177 a year for.

The school met unions last week and while progress was made in discussing alternative options for pensions arrangements, no offer was made to justify withdrawing strike action, the NASUWT, which said it is committed to further talks, said.

Earlier this month, teachers in the EIS union were the first to take strike action in the school’s history over the dispute.

Dr Patrick Roach, NASUWT general secretary, said: “It is regrettable that it is only at the 11th hour that the school agreed to sit down with us to discuss a resolution to this dispute.

“It is clear that it is only the refusal of members to accept being treated in such a shoddy manner that has brought the employer to the table.

“While we remain committed to working with the employer to resolve this dispute, until such time as concrete proposals come forward from the employer, the planned strike action will continue.

“We have made it clear to the employer that we are ready to meet at any time to continue talks and hope that concrete proposals can be brought forward by the school that will safeguard our members’ pensions and enable us to resolve this dispute.”

Mike Corbett, the NASUWT’s national official for Scotland, said: “Members at Hutchesons’ have made the very difficult and brave decision to stand up for their rights in the face of immense pressure and hostility from their employer.

“It is disappointing and frustrating that the employer has not acted before now to open talks with us that could have avoided industrial action and the consequent disruption to pupils and parents.

“If the employer is serious about wanting to resolve this dispute it must now bring forward detailed and credible plans for teachers’ pensions that can start to rebuild the trust and confidence of staff.”