Half of French schools could close Thursday as up to three-quarters of teachers walk out, pushing back against the government after three changes of Covid rules for classrooms in the space of a week.
The strike "demonstrates the growing despair in schools", the largest teachers' union Snuipp-FSU said in a Tuesday statement announcing the strike.
They complain that their members are unable to teach properly, are not adequately protected against coronavirus infection and frequently hear about changes to health precautions via the media rather than from higher-ups.
Coming as France's presidential election campaign gets under way ahead of an April vote, the walkout is uncomfortable for a government that has prided itself on keeping schools open to ease pressure on parents through the pandemic.
With many pupils off sick and difficulty combining distance learning with in-person classes, "it's not school that's open, but a kind of 'daycare'," Snuipp-FSU said.
President Emmanuel Macron's presidential election challengers have seized on the strike.
"I feel the enormous pain both of the school community and especially of parents," conservative candidate Valerie Pecresse told France 2 television.
Motivated by long queues for tests outside pharmacies, the government this week eased rules on Covid checks for students who have been exposed to an infected person, with Prime Minister Jean Castex announcing the changes on Monday's evening news.
"We've listened and made changes," government spokesman Gabriel Attal told reporters after a cabinet meeting Wednesday, acknowledging "weariness" among both parents and teachers.
The shift up the chain of command appeared to be a reaction to the anger teachers direct at Education Minister Jean-Michel Blanquer.
He has not backed down from tough talk, saying on television Tuesday that "you don't go on strike against a virus".
But his management of the pandemic response in school has prompted a total of 11 unions to participate in Thursday's walkout, in a rare show of unity that could make for the biggest strike in decades.