President Francois Hollande promised as part of his election campaign last year to add a half day of classes on Wednesday - currently a day off - and shorten the school day for the rest of the week.
His argument was that French kids' education was suffering because they had one of the shortest school years in Europe but the longest school day.
But teachers are worried that the extra half day - due to be introduced as from September this year - will add to what they see as their already heavy workload without any financial gain.
Jerome Lambert of the teachers' union, SNUipp-FSU, noted that teachers, a majority of whom are Socialist supporters, were "disappointed" by what he said was a reform that in reality was no more than window-dressing.
"We need to reopen the debate," he said.
Nicolas Wallet, a teacher from a central Paris school, said the government's promise to provide sporting or cultural activities to make up for the shortened school day was not credible.
The education ministry has proposed a range of activities to fill the extra time children would have after school if the reform goes ahead.
But that could require extra staff and more funding from budgets already strained by the economic crisis gripping France.
The striking teachers planned to hold a protest march later on Tuesday.
French primary schools have had a mid-week break since the 19th century.
But the state provides low-cost "leisure centres" - which are usually in schools - where working parents can send their children on Wednesdays.
Following his election in May, Hollande vowed to make education a key focus of his five-year term.
He proposed reducing the number of students forced to repeat grades, increasing teacher levels, schooling children at younger ages in disadvantaged areas and boosting measures to fight absenteeism.