Teachers at the former school of First Minister Humza Yousaf have walked out on strike in a row over pensions.
Some 78% of members of the EIS teaching union voted in favour of action at Hutchesons' Grammar in Glasgow.
The union said staff were sent letters saying they would have to sign new contracts, with the jobs of anyone who refused being advertised elsewhere.
The school said it was "extremely disappointed" at the strike and that it was trying to protect teachers' jobs.
It is the first strike action in the history of the private school - which boasts both Mr Yousaf and Scottish Labour leader Anas Sarwar among its former pupils.
