When I was twelve years old, my father took a sabbatical in London, and our family moved from quiet, middle class Iowa City to London's buzzing if somewhat downtrodden Camden neighborhood. I enrolled in a "state school" called J.F.S. (Brits confusingly call their private academies "public schools".) I would walk to school by myself past the gloomy "estates" (housing projects) and the jumbled little shops of Kentish Town, shyly self-conscious in my blue blazer, gray sweater, and striped blue-and-gold tie.

But I would not put on my blue-and-gold beanie until I arrived at the school. A beanie is an unusual accessory in an English school uniform. Its significance is suggested by the full name of the school, Jews Free School. The "beanie" was a yarmulke.

Britain lacks the strict separation between church and state of the U.S. A third of its state-funded schools are religious. Most are Anglican or Catholic, but 37 are Jewish, seven Muslim, two Sikh, one Hindu, one Greek Orthodox, and one Seventh Day Adventist. The majority of my classes at J.F.S. were secular, but Hebrew study and a course called "Religious Knowledge" were required. The cafeteria was kosher, and we recited Jewish prayers several times a day. In addition to the state holidays, we also got the Jewish holidays off, and the school closed early on Fridays--my favorite part of attending a Jewish school.

J.F.S. is the news today. Britain's Supreme Court has ruled that it must change its admissions policy. J.F.S. only admits Jews, but that practice is not illegal in the U.K. What is illegal is the way J.F.S.'s admissions policy defined what it means to be a Jew. According to traditional Jewish law, if your mother is Jewish, you're Jewish, no matter what you believe. If she's not, then you're not, no matter what you believe. You can, however, convert to Judaism, though it's not nearly as easy as dunking your head and embracing God. There are various rituals and tests of knowledge required, and if you're male, you need to be circumcised. (The last requirement may partly explain why there are so few Jews in the world.)

The case in the news involves another twelve-year-old boy, known as M in court papers. His father is Jewish, his mother is Jewish, and I presume that M is circumcised. But M's mother was not born Jewish; she converted in a progressive Jewish synagogue. Since J.F.S.'s admissions policy only recognized Orthodox Jewish conversions, M's mother's conversion didn't count. Ipso facto, the school did not consider M to be Jewish and refused to admit him.

M sued. He lost. He appealed. The Court of Appeal found the school's admissions policy to be based not on religion, which is legal, but race, which is not, and ruled for M. J.F.S. appealed. In a 5-4 decision, the Supreme Court upheld the ruling. J.F.S. has changed its admissions policy to use a "religious practice test" instead of the matrineal test.

The question of what it means to be a Jew is one of the most divisive questions within the Jewish community. Orthodox synagogues and organizations claim exclusive privilege in defining Jewish identity. For instance, under Israel's Law of Return, all Jews are allowed to settle in Israel and to become citizens. For most of Israel's history, the Chief Rabbinate of Israel had the authority to determine the validity of conversions according to Orthodox requirements. In 2005, the Israeli Supreme Court ruled that non-Orthodox conversions outside Israel were also valid.

I have a younger sister who was too young to attend J.F.S. when we lived in London. She is adopted. As a Native American, we're pretty sure that her biological mother wasn't Jewish, Mormon theology notwithstanding. When I was young, I remember traveling to Des Moines, where my sister had a ritual bath called a mikvah and officially converted to Judaism under Orthodox rules. My parents explained that our synagogue, the only one in Iowa City, wasn't religious enough and that my sister needed the full conversion in case she someday decided to move to Israel or wanted to marry an Orthodox Jew. They weren't thinking about Jewish schools in England.