New York City public high schools with academic requirements for entry offer a rigorous education, but admission is supposed to be based on performance, not on income. So why do so few low-income students manage to get in?

A look at two dozen of the city's most elite high schools -- those that require a score of proficient or higher (a 3 or 4) on both the state math and reading exams and those that require a spectacular score on the "specialized test" -- shows a huge discrepancy in who's enrolled, based on income.

About 74% of high school students citywide qualify for free or reduced lunch ($41,348 or less for a family of four) compared with 41% of students at the exam and selective schools.

Education advocates say the results show that the city's elementary and middle schools are not preparing all students equally – and that it is very difficult for schools, by themselves, to overcome the disadvantages of poverty.

"The data shows that all students do not come into the school system with an equal chance of accessing the most coveted programs," said Kim Sweet, director of Advocates for Children.

The selective schools promise not only a good high school education – the average graduation rate is a whopping 94% - but are also a gateway to some of the country's top colleges. And it's not just poor kids who miss out.

English language learners make up 14% of student bodies in city high schools, but comprise only 1% at the exam and screened schools. The average population of special education students in high schools is 14% compared with 4% at the schools that require high exam scores for entry.

The schools are stratified by race as well. City high schools are 38% African American and 44% Latino, compared with 16% and 19% respectively in the elite schools.

Education Department officials note that nationally students from low-income backgrounds don't do as well in school as their more affluent peers. They say they are trying to create more high-quality schools so more students can attend screened schools if they wish, a goal which guides their policy of shuttering low-performing ones.

"It's not about changing the formula," said Education Department spokesman Frank Thomas. "It's about improving student outcomes so they are qualified when they arrive."

A few of the selective high schools do veer from the norm. Among them is the Bronx High School for Medical Science, where 87% of the students qualify for free or reduced lunch, 93% are black or Latino and 9% receive special education services. Unlike most selective schools which require students to have at least an 80 average in academics, Medical Science admits students with a 75 grade point average. About 79% of students at the Manhattan Center for Science and Mathematics are from low-income families. Some experts question the validity of using standardized exams as the main criterion for selecting students and advocate more inclusive policies. 

"Otherwise, when you have a special educational opportunity like these schools," said Kevin Welner, professor and director of the National Education Policy Center at the University of Colorado at Boulder, "very often what we see is a version of the rich get richer; that is, the more prepared get better prepared."