Lila and Alma Levy, aged 18 and 16, relaunched the divisive public debate at the start of term when they refused to remove their hijabs in class.
They were permanently excluded in October from the lycee Henri-Wallon in Aubervilliers, a Paris suburb.
The wearing of "ostentatious" religious symbols is regarded as breaking France's code of secularism in public life. Education is at the heart of traditional republican values. Under the constitution state schools must be strictly secular, and religiously, philosophically and politically neutral.
The problem with Islamic headscarves first arose in 1989 when a school head excluded three girls for wearing them. Under a State Council ruling following that case and subsequent circulars, pupils may display religious signs, for example by wearing a small crucifix, so long as they are not ostentatious, do not proselytise, provoke or propagandise or interfere with the freedom of others.
Pupils are also forbidden from interfering with the curriculum by, say, refusing to take part in lessons such as sports or biology. Schools are left to interpret this with local courts ruling in disputed cases.
President Jacques Chirac and premier Jean-Pierre Raffarin are among politicians and other public commentators, who to varying degrees, support a law to impose secularism in schools.
But others, including interior minister Nicolas Sarkozy, are against legislation, and teachers from the excluded girls' lycee say their case should not be used to justify a legal ban on wearing hijabs in school.
A commission examining secularism in public life under state Ombudsman, Bernard Stasi, will report at the end of the year and could lay the foundations for legislation banning headscarves.