The walkout is "a warning to the government" about teachers' "daily life, their suffering at work and the lack of recognition, especially in their pay," said primary school teachers' union FSU-Snuipp, predicting "hundreds of schools will be closed".

The union added that "the situation has been inflamed by the nomination of a part-time minister who has forfeited her credibility".

With former education minister Gabriel Attal promoted to prime minister, Amélie Oudéa-Castéra was given the key education brief alongside sports, including this year's Paris Olympics, and youth.

Thursday's strike, which coincides with ongoing protests by agricultural workers, had been planned since before the government reshuffle that put Oudéa-Castéra in place.

But she set teachers bristling from the moment of her nomination, as she claimed she had put her son into an exclusive Catholic private school because of "loads of hours with no proper replacement" teacher at his state primary.

The then-teacher of Oudéa-Castéra's son later came forward to contest her version of events, while the press have also unearthed allegations of sexism, homophobia and circumventing competitive university admissions processes at the school.

"I have mixed feelings, always anger, for a long time, but also exasperation and incomprehension," said Benjamin Marol, a middle school history-geography teacher from Montreuil east of Paris.

He complained that the government is toying with ideas like imposing school uniforms and dividing classes by ability, rather than tackling more fundamental issues.

Teachers had returned from the holidays to "yet another change of pilot and... the nomination of a minister who had a catastrophic start," said Elisabeth Allain-Moreno, secretary-general of teachers' union SE-Unsa.

Some 47 percent of middle and high school teachers were on strike Thursday morning, the leading Snes-FSU union said, while FSU-Snuipp tallied 40 percent in primary schools.

A Paris march will kick off towards the education ministry from 2:00 pm (1300 GMT), while others were planned in major cities nationwide like Marseille, Rennes and Nantes.

"I'll be on the street to express my profound disagreement with... what the minister said about public schools," said Anne, a maths teacher from Nice, who did not give her surname.

"I feel wounded and humiliated by a minister who's completely out of touch," she added.

Oudéa-Castéra's triple role even has some members of President Emmanuel Macron's party in parliament doubting.

"What's hard is we managed to win some credibility in school staff rooms and that was gone in three days," said a senior lawmaker, who asked not to be named.