Some 100,000 protesters marched through Jerusalem with the 40 couples, who planned to hand themselves over to the police in compliance with a Supreme Court ruling.
About 10,000 police officers were mobilised in the city.
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I don't want my daughter to be educated with a girl who has a TV at home
Yakov Litzman, MP United Torah Judaism party Israeli press: School dispute
Marchers brandished placards and banners. "The Supreme Court is fascist," one poster read.
One protester, parent and rabbi, Meir Elmaliach, told the crowd from a makeshift stage: "We are strong because God is with us."
A similar protest in Bnei Brak near Tel Aviv drew about 20,000 people, said police.
The families at the centre of the legal battle come from a strictly observant sect of Hasidic Jews called Slonim, who have Ashkenazi lineage.
'Not a drop of racism'
They have pulled their children out of Beit Yaakov girls' school in the West Bank settlement of Immanuel, and set up lessons elsewhere in the area.
The Slonim parents say their objections are based on differences in religious observance between the Ashkenazi and Sephardi traditions.
Yakov Litzman, an MP from the ultra-Orthodox Ashkenazi party, United Torah Judaism (UTJ), told army radio there was "not a drop of racism" in the parents' decision.
"There is a set of rules [in the ultra-Orthodox community]. We don't want televisions in the home, there are rules of modesty, we are against the internet," Mr Litzman was quoted as saying by AFP news agency.
"I don't want my daughter to be educated with a girl who has a TV at home."
The court had given the par