Some teachers at a Gwynedd school are staging a second day-long strike in protest at pay and job cuts.
The union NASUWT claims the local authority and school governors were being "intransient " over proposed redundancies at Tywyn Secondary School.
The union is warning the dispute has potential to spread to other schools across the county during the summer months if it is not resolved.
Gwynedd council has been asked to comment.
The teaching union said it had put a 10-point plan to the school's governing body to resolve the dispute which affected six teachers, around a quarter of the teaching staff, two of whom face redudancy.
Union general secretary Chris Keates said: "This was rejected. Instead the governors opted to push forward with redundancies."
She added: "To sack teachers or cut their pay when the school budget is in surplus is appalling.
NASUWT Wales Organiser Rex Phillips added: "The NASUWT has made strenuous efforts to attempt to resolve this dispute but it is now clear that we are being blocked by the intransient attitude of the school governors and local authority."
A picket at the school, which around 300 pupils, was held on Thursday morning.
The union's first one-day strike was held at the school on 29 June.