Mayor Bloomberg got the whole city talking by assigning letter grades to the public schools. Some suggested charter schools should be next, and they were. Left out, until now, have been private schools.

The New York Sun has assigned its own letter grades, using a mathematical formula that takes into account the school’s net assets and the number of students it sends to Harvard and adjusts for the size of the student body.

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At the top is an all-girls school on the Upper East Side known as one of the best schools in the city, Brearley. It was the only school to earn an “A+” grade on the Sun’s survey. In the past three years, 12 Harvard freshmen hailed from Brearley — a high number for an upper school of just 207 students. The school reported more than $100 million in net assets, the third-largest number in the city, in a tax filing for 2005, the most recent year for which data from all the schools was available. That is more than double the assets of many included schools.

New York State is unusual in that 20% of students here do not attend public schools. In certain parts of the city, the rate is even higher. According to U.S. Census figures, 85% of children in a census tract near the Guggenheim Museum attend private schools.

“Almost every variety of school is represented in New York City,” the executive director of the New York State Association of Independent Schools, Elizabeth Penney Riegelman, said. “There are far more options than there are in virtually any other city.”

Navigating all the choices, however, can be difficult. Aside from regular evaluations required to maintain accreditations — performed by one of two independent oversight groups, Ms. Riegelman’s NYSAIS and the Middle States Association of Colleges and Schools — many schools shun rankings and any information-sharing that would facilitate it. They are also tight-lipped about what happens behind their doors, training even alumni not to speak to reporters.

Yet the public record offers at least one measure of how the schools are doing, and how they stack up against one another.

As nonprofit organizations, schools must file tax forms each year reporting their assets, liabilities, and net worth. As college-feeders, they also create substantial paper trails. To find out how many Harvard graduates a school produced, for instance, one need only check the university’s freshman register — known on campus as the facebook — which lists nearly every class member’s name and high school.

A new ranking system drawn up by The New York Sun this year, the New York Sun Private School Index, draws on these two data points, adjusting to account for each school’s size.

Harvard admission is used not because it should be every student’s goal but because, being so elusive, it is a good stand-in for a graduate’s odds of going anywhere he or she pleases. Last year only 9% of applicants — a self-selected group to begin with — got into Harvard.

The net assets number, which includes a school’s endowments and real estate, gives a sense of a school’s stability and vitality.

Different schools may account for assets differently, but having a deep financial reserve can be an advantage. “If you’re operating at a level where your net assets is equal to say one month of operating expenses, then you’re running awfully close. It wouldn’t take much to really upset the organization’s finances,” an accounting consultant based near Washington, D.C., Christine Manor, said. “If however your net assets are equivalent to nine months of operating expenses, then you have more of a cushion.”

The author of a guide to Manhattan independent schools, Victoria Goldman, said asset and endowment figures are good indicators of a school’s ability to “weather hard times.”

By communicating the size of the endowment, net assets tell more complicated stories: how much donor loyalty a school has built up, the quality of a school’s facilities. and how easily it can pass out financial aid.

The Sun rankings — not the only way to look at the schools, but one way to assess them — confirm some reputations and offer some surprises, too.

Three members of a top tier of schools known as the Ivy Preparatory School League are the Sun’s top three.

After Brearley comes its brother school, Collegiate. An Upper West Side coeducational school that is nearing its 300th birthday, Trinity, came in third. Both schools sent many students to Harvard and had net assets above $50 million.

In a push that could add new fuel to longstanding competition among the city’s top girls’ schools, two Brearley rivals, Spence and Chapin, get two of the last A’s at Nos. 6 and 4 respectively.

The Sun list de-emphasizes other well-regarded schools, such as the Saint Ann’s school in Brooklyn Heights, which, with $24 million in net assets and only five alumni recorded in Harvard’s freshman register, got a low B. The head of school at Saint Ann’s, which topped a 2004 ranking by the Wall Street Journal, dismissed the Sun’s ranking system. “We don’t believe in grades,” Lawrence Weiss said. “I conscientiously object.”

Some schools that were not included in the rankings for lack of data nonetheless fared well in particular measures. A free Catholic school for boys, Regis High School, was the alma mater of 10 students listed in the Harvard registers. The school was not included in the rankings because, as a religious institution, it does not have to report its net assets.

Five schools received “D” grades in the Sun rankings, the lowest handed out. All of the “D” schools had no graduates listed in the Harvard registers. They all reported net assets below $30 million.

Cynthia Bing, the head of the school advisory service for a group that helps parents navigate local independent schools, the Parents League, said parents should not make decisions solely on letter grades or numerical rankings based on Harvard admissions or net assets. “Assessing a school is much more complex, and it should be very individualized for each student and each family on their best match,” she said. “The gut is the best measurement. That’s how parents should rate schools: they go visit them, or if the kids are older, they go visit.”

When the city Education Department handed out grades to most of the public schools under its watch this year, many parents and principals condemned the judgments for being based almost entirely on standardized test scores. But a recent Quinnipiac poll showed most New York City parents who know the grade their children’s school received, 75%, believed the grades were fair, compared to 21% who didn’t.

A deputy schools chancellor, Christopher Cerf, declined to comment on whether grades are appropriate for private schools, but he defended the general concept. “As a parent, I think the more folks know about their schools, the better they’re able to make sound choices,” he said.

The first-ever New York Sun guide also includes superlatives for many of the schools included on the list as well as some that were not. The superlatives — ranging from best humanities program to best cafeteria — were compiled from interviews with dozens of parents, teachers, administrators, and alumni.

The list of 23 schools that received grades is based on which schools provided information on their net assets to the Internal Revenue Service in 2005 and the total number of students they serve either to a publicly available online guide, Peterson’s, or directly to The New York Sun.

The director of admission at Harvard, Marlyn McGrath, called New York schools an excellent group across the board, but said none of them is the key to gaining entry to her college. “We have excellent students who have been to more modest schools, and we have more modest applicants who have been to very excellent schools,” Ms. McGrath said. “We don’t admit schools; we admit students.”