A day after Schools Chancellor Joel Klein vowed to crack down on teachers who sexually exploit their students, another teacher was arrested on charges of sexual abuse and endangering the welfare of a minor.

The teacher, Joanna Hernandez, 27, was arrested for allegedly embracing and kissing open-mouthed a 15-year-old student at her school, I.S. 55, in Ocean Hill-Brownsville.

A spokesman for the United Federation of Teachers, Ron Davis, said Ms. Hernandez has no history of molesting youngsters and her colleagues are “shocked” by the arrest. He said Ms. Hernandez is known as an “outstanding educator.” According to the union, she was invited two years ago to the National Mathematics Conference in San Antonio to present an innovative technique for teaching math to special-education students.

“Ms. Hernandez has a reputation in the school for being an excellent math teacher and a very hard worker, and she vehemently denies the charges being made against her,” the union president, Randi Weingarten, said. “If the charges are proven to be true, then she should not be in a classroom. But no one should rush to judgment before she has had a chance to avail herself of her due process rights.”

According to the Department of Education, Ms. Hernandez started at the Brooklyn public school in September 1999 as a regular substitute teacher. She became a math teacher in 2001. Her current salary is $45,583.

Ms. Hernandez’s arrest comes a week after New Yorkers learned that another teacher was pregnant by an 18-year-old student. Last week, there were also reports of a guidance counselor having an affair with a 17-year-old student and a middle-school teacher flashing school girls.

Although kissing a minor might not seem as serious a crime as some of those alleged last week, experts said it could be just as upsetting to a student.

An education professor at Hofstra University, Charol Shakeshaft, said any sort of unwanted physical contact by teachers could be devastating to children. Ms. Shakeshaft, who recently wrote a study for the U.S. Department of Education on sexual exploitation of students by educators, said: “Targets of sexual molestation are more likely to have drug and alcohol problems later in life, are more likely to commit suicide … and have difficulties developing and sustaining healthy relationships.”

She said some research shows the impact can be most traumatic when the victim is between 11 and 14, but she said abuse can harm children of all ages. She said 7% of students in nationwide surveys report that they have been sexually exploited by a teacher between kindergarten and 11th grade, but only 10% of those students say they reported the incidents.

The special commissioner of investigation for the city school district, Richard Condon, said his office has received 180 complaints this year about sexual abuse by teachers, and it has substantiated claims against the teachers in 30 cases.

His office does not investigate cases that are spearheaded by the Police Department, as Ms. Hernandez’s case has been.

The number of cases has not risen in the past five years, he said, but that it hasn’t dropped is troubling.

“I think it’s a major problem because it’s adults in a position of authority taking advantage of children,” Mr. Condon said. “It troubles me because of the nature of the complaints and the fact that it hasn’t gone down.”

On Tuesday, the chancellor reminded principals in his weekly newsletter to school leaders that “abuse of this sort is absolutely unacceptable.”

“We can have no tolerance for anyone who would harm a child,” Mr. Klein wrote.

He told principals to remind their staffs that lewd remarks, unseemly email messages, inappropriate touching, and sexual and romantic relationships have no place in schools. He said he has supported legislation at Albany that would make it a criminal offense for a Department of Education employee to engage in sexual contact with any student, and he said he was working with state lawmakers to seek additional legislation that would require the dismissal of any employee found by the special commissioner of investigation to have engaged in sexual activity with a student.

Ms. Weingarten said Mr. Klein’s proposals show that he wants to “fight,” rather than work out a solution through the contract-negotiation process.

Yesterday, the chairwoman of the City Council’s Committee on Education, Eva Moskowitz, said: “If we need a law against teachers having sex with their students, we are in tremendous trouble. We need to hire people who understand appropriate student-teacher relations. Clearly, there is a problem with the hiring in the New York City school system.”

In an interview, the state senator who proposed the legislation that would make sexual contact with students a crime, Carl Kruger, said the chancellor hasn’t been able to stop teachers from molesting their students and called on Mr. Klein to step down.

“We’re talking about a system that’s out of control, that’s broken,” the Brooklyn Democrat said. “The abuses that we see now time and time again – we see teachers pulled out in chains from our schools – point to a system that has no leadership. I think it’s about time that the chancellor resign.”

Also yesterday, the Police Department announced that another teacher, Robert Walker, was arrested at Essex Street for possessing illegal “gravity knives.”

When he was arrested, police found that there was an open warrant for his arrest on a 1993 charge of disorderly conduct.

Mr. Walker, 53, was also arrested in 2001 and charged with resisting arrest, obstruction of governmental administration, and possession of marijuana, according to the authorities. The shop teacher, whose salary is $69,359, has been removed from his school, Transit Tech High School, according to the Department of Education.