“Once they become automatic, we’ve really hurt that child’s chances to receive a high school diploma,” he added “We’ve got to find ways to keep those kids in school. Don’t get me wrong — we have to provide safe environments for all the other kids. But you have to balance it out and cut down the suspensions and expulsions.”

Almost 15 percent of students, a vast majority of whom had extensive school disciplinary files, had at least one record in the juvenile justice system, according to the report.

Minority students facing discipline for the first time tended to be given the harsher, out-of-school suspension, rather than in-school suspension, more often than white students, the study said. (The nature of the offenses was not noted.) A disproportionate number of minority students also ended up in alternative classrooms, where some have complained that teachers are often less qualified.

“What we really need to do is go in to those districts and see if these really are choices being made,” Mr. Skiba said. “We don’t really know enough about the reasons for African-American and Latino over-representation in school discipline. We have enough data to show that it’s more than just poverty and any greater misbehavior. My guess is it’s very subtle interactional effects between some teachers and students.”

Mr. Thompson, of the Council of State Governments, said one of the study’s most important findings was how demographically similar schools disciplined students differently. Although Texas law requires suspension or expulsion for certain offenses, Mr. Thompson said that 97 percent of suspensions were discretionary, and that suspension rates might say as much about administrators’ discipline philosophy as about student behavior.

“Schools are making very different uses of school discipline,” he explained. “And they can have an impact on how often a kid repeats a grade or graduates. We need to recognize that it’s something we need to improve upon.”

While the study found links between school discipline and criminal activity, there is no way to know whether one caused the other. Educators have long complained that many students, particularly from poor families, arrive in classrooms with problems far beyond academics that they have few tools to control.