Representatives of school staff said the guidelines, which apparently try to ban Muslim veils and Jewish skullcaps without outlawing other headwear such as Sikh turbans, were so vague they would lead to endless disputes between pupils and schools.
In March France passed a ban on the hijab (headscarves), skullcaps and large Christian crosses amid criticism from religious groups that saw it as an attack on religious freedom. The guidelines were meant to spell out how schools should enforce the ban.
"This text offers many ways to twist the law," Philippe Guitter of the main school principals' union told the daily Le Monde. "We risk ending up with what we wanted to avoid - endless negotiations with the pupils."
"Everybody will have a different interpretation of these guidelines," teachers' union activist Patrick Gonthier said on France 2 television.
Principals and teachers strongly pressured the government to pass an outright ban on all religious gear in state schools, arguing they needed it to stem a growing tide of religious tension and violence, especially between Muslims and Jews.
Muslim groups responded with pleas for a flexible application of the law. France's small Sikh community rallied international support to help them keep their turbans.
The ban was criticised from abroad, especially by Muslim countries.
"This text offers many ways to twist the law"
Union chief, France
The guidelines, which appear to try to relax it for schools in France's majority Muslim island of Reunion in the Indian Ocean, say pupils could wear "traditional clothing" or religious items if they are not meant to display one's faith.
Schools would be free to ban all head coverings or allow girls to replace full headscarves with bandanas, one compromise proposed by the Union of French Islamic Organisations (UOIF).
Guittet said the principals and teachers would not approve the draft in negotiations due to be held between now and their planned ratification by education authorities on 6 May.
"If this draft isn't changed, I will ask my members to ignore these guidelines and simply apply the law," he said.
By contrast, the guidelines clearly state that no pupil will be allowed to skip sports or science classes because their religion does not agree with the subject matter. Some Muslim pupils snub these classes as "un-Islamic".