Black students, who make up 30 percent of the schools’ enrollment, accounted for more than half of all suspensions every year from 1999-2000 to 2008-9, the period covered by the report. Special-education students, who make up 16.2 percent of the enrollment, served about one-third of all suspensions. There are about 1.1 million students in city schools, and the suspension rate, roughly 1 for every 14 students, is similar to that found in nationwide studies.

Donna Lieberman, executive director for the New York Civil Liberties Union, said suspensions could have serious consequences to a student’s education. A national study found that students who had been suspended three times or more in their sophomore year of high school were five times as likely to drop out or graduate late. In New York City, one in five students suspended in a single year is suspended at least twice in one year, the report showed.

“The growing reliance on suspensions in New York City schools all too often denies children, often the most vulnerable and in need of support, their right to an education,” Ms. Lieberman said.

Last year, the Education Department revised its disciplinary code to decrease the number of offenses that warrant mandatory suspension, known as zero-tolerance infractions, to 21 from 29, as it had been since 2007.

There were only seven zero-tolerance offenses listed in the code in 1998.

Changes have also been made over time to the definition and scope of some of the infractions. Natalie Ravitz, an Education Department spokeswoman, said the changes were to conform with the times and to give principals more latitude. The department is looking to use more social and emotional mediation to address behavioral issues among special-education students, she said.