“A tale of two cities.” “Wrong.” “A police state.” “Alarming."

Four Democratic candidates for mayor on Monday used such terms to denounce elements of student discipline in New York City’s schools, saying black and Latino students’ suspensions were disproportionately high and principals imposed unnecessarily harsh penalties on students with disabilities.

William C. Thompson Jr., a former comptroller now running for mayor, suggested that the city should emphasize mental health counseling over punishment.

“We’re suspending students far too often,” Mr. Thompson, who was the Democratic nominee for mayor in 2009, said at a City Hall news conference organized by Dignity in Schools, an advocacy group.

John C. Liu, the current comptroller and also a mayoral contender, said school leaders had overstepped their authority, creating a “criminal-industrial complex.”

Two Democratic candidates, Bill de Blasio, who is the public advocate, and Christine C. Quinn, the City Council speaker, issued statements on the subject.

In the 2011-12 school year in the city, 52 percent of suspensions involved black students and 36 percent involved Hispanic students, the Education Department said. Over all, 40 percent of the public school system’s students are Hispanic, 27 percent are black and 15 percent are white.

At a hearing on student discipline before the City Council on Monday, Kathleen Grimm, a deputy schools chancellor, defended the department.

Ms. Grimm said that since the beginning of this school year, suspensions of black male students had decreased 26 percent and suspensions of Hispanic male students had decreased by 25 percent. She pointed to efforts to simplify the disciplinary code and promote the use of restorative justice tools.

“Promoting a positive school culture and improving school safety has been and continues to be a cornerstone of the department’s efforts,” she said.

While acknowledging that the number of arrests and suspensions has fallen over the past several years, Mr. Thompson and Mr. Liu argued that the city should focus on alternative means of punishment.

“Parents don’t want to see school discipline outsourced to the police or to the emergency room,” Mr. de Blasio said in a statement.

Ms. Quinn said in her own statement, “Continuing the practices we know are harmful to students and their futures will not work.” She said “we need to add alternatives to suspensions and arrests.”

Student discipline has long been a contentious topic in New York City. Critics of the Education Department’s practices hope they can harness some of the political energy that has fueled anger at the Police Department’s stop-and-frisk tactic, which has also been criticized for disproportionately affecting minorities.

With five months until the city’s mayoral primary, the candidates are making a point of highlighting their education credentials.

On Monday, Mr. Thompson noted his stint as president of the city’s Board of Education, which at the time oversaw the city’s schools, and Mr. de Blasio pointed out that he was a parent of a high school student.

The candidates are aggressively courting the endorsement of the city’s powerful teachers’ union, the United Federation of Teachers. Union representatives will meet on Wednesday to decide whether to offer an endorsement in the primary.