Despite years of studying, all they were exposed to was religious texts and their interpretation, leaving them clueless about the basics of the national curriculum.
"I once heard them talk about the theory of evolution, very furtively, in the yeshiva when someone said certain people think they are descended from the apes," said 26-year-old Yaakov Fink, a former religious scholar.
"It triggered a gale of laughter and the rabbi said that anyone who believed that must be a monkey," said Fink, who now studies psychology.
In place of the trademark ultra-Orthodox attire of sidelocks, skullcaps and long black coats, he is now dressed in sporty casual wear.
Nothing in his appearance or speech gives a clue that this computer enthusiast who loves to have a beer with his friends spent the first 21 years of his life as an ultra-Orthodox Jew, cut off from modern thought and learning.
In the yeshiva world, he says, everything is designed "to ensure that no doubt, however small, can sneak in" that could challenge blind faith.
No room for doubt
Yet nagging doubts did crop up for this young seminarian -- and they never went away.
"It happened that one Saturday night after the end of shabbat I simply couldn't bring myself to go back to the yeshiva -- and I never did understand why," he told AFP.
Although leaving the ultra-Orthodox world was hard, there was worse to come as Fink struggled to integrate into the secular -- and elitist -- university system with extremely limited skills.
In maths, he had the ability of a 10-year-old, his English went no further than the alphabet and he had absolutely no idea about history, geography or science.
What helped him was getting involved with a group called "Out for Change", which counts around 300 members -- all of them former ultra-Orthodox.
Formed with the express aim of taking legal action against the state, the organisation also runs courses for people who want to integrate into modern society.
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But its members are mainly engaged in a lawsuit seeking damages from the state, which provides 75 percent of the funds for ultra-Orthodox education without demanding any supervision over what its 400,000 students learn.
"What did we study in the yeshiva? The Torah and its commentaries and that's about all," Fink said, referring to the first five books of the Bible.
Ultra-Orthodox girls have their own single-sex schools, and do learn lay subjects such as maths, history, sciences and languages, but not to a sufficiently high level to qualify for universities -- coeducational institutions which contravene the ultra-Orthodox requirement for female "modesty".
Keeping young women out of such an environment is a deliberate policy of the pious, say those who have left.
"Yeshiva education has nothing in common with Western education. It's like what you would have found in eastern Europe more than 200 years ago," Yossi David told AFP.
"Its ultimate goal is to not evolve," said David, who finally graduated from high school-level studies at the age of 25 thanks to evening classes he paid for by working around the clock.
"I was deprived of knowledge in all areas: critical thinking; reasoning ability; writing and creativity."
He did the same to prepare for his entrance exams for Jerusalem's prestigious Hebrew University.
Now 32, he teaches and researches in the field of political communications.
His journey to enlightenment was punctuated by major revelations -- like the day it dawned on him that the biblical Hebrew he had grown up speaking was not the same as modern Hebrew used by other Israelis.
If today such recollections make him smile, the thought that tens of thousands of children -- with the complicity of the state -- are denied a basic education really "hurts," he says.
"We know it will be very difficult to get real justice for all the children," David said.
"But at least we want to try."