Teachers of color continue to be underrepresented in Philadelphia and across the country, according to a study released Wednesday.

In particular, the number of black teachers in nine large cities dropped between 2002 and 2012, researchers for the Albert Shanker Institute found.

That means many school districts - including Philadelphia's - aren't doing enough to recruit and retain a diverse teaching force, union officials said.

Three of every four Philadelphia students were black or Hispanic in 2012. Just over 1 in 4 teachers was black or Hispanic.

"This report is an indictment," said Randi Weingarten, president of the American Federation of Teachers. Researchers for the nonprofit institute, which is connected to the national teachers union, examined teacher diversity in Philadelphia, Boston, New York, Los Angeles, Chicago, Cleveland, New Orleans, San Francisco, and Washington.

"It's important to have a diverse workforce, because the teachers and staff in schools should look like America," said Jerry Jordan, president of the Philadelphia Federation of Teachers.

A diverse teaching force benefits all students, research suggests, but in particular, students of color need to see "people who look like them who have earned degrees, who have professional positions, and who understand their culture," said Jordan.

Weingarten said white students benefit just as much.

"We need to defy stereotypes," she said. "We need to debunk myths."

The report singled out several trends that officials said they found disturbing:

"Minority educators have distinctly higher quit rates - turnover rates - than do nonwhite teachers," said Richard Ingersoll, professor at the University of Pennsylvania's Graduate School of Education and a researcher on the report.

The problem lies with retention. Generally, over the last several decades, there has been a significant increase in the minority teaching force, though the percentage of minority students in U.S. schools far outstrips the percentage of minority teachers.

And while minority teachers tend to be concentrated in urban areas and in schools serving poor children from minority communities, the report found, teachers leave not because of the needs of students they serve, but because of working conditions, the report said.

"It goes to the lack of support in buildings, the lack of resources in order to retain teachers," said Jordan. "Teachers don't have the tools to do their jobs, and they often don't have safe environments."

Union officials and Ingersoll said it was not entirely clear why minority teachers left the profession at higher rates than their white peers.

In 2009, the conclusion of a decades-long court case ended the Philadelphia School District's "desegregation requirement," which mandated some racial balance in school staffing. That has made a difference in how teachers are distributed, Jordan said.

Programs such as ones that encourage interested district paraprofessionals - people who often come from the communities they work in - to qualify as teachers could help the problem, Jordan suggested. One used to exist in Philadelphia but has been discontinued.

Weingarten, of the AFT, also called on President Obama to hold a White House summit on teacher diversity. "Let's get the best minds in the room," she said.

Philadelphia officials, in a statement, said they value teacher diversity.

"We continually review our recruiting practices so that we improve outreach," schools spokeswoman Raven Hill said. "The school district is making more concerted efforts to increase our bilingual staff as well. As with many school systems nationwide, our pipeline is not as diverse as it should be."