The New York City comptroller, William C. Thompson Jr., released a report on Tuesday suggesting that the city’s graduation rate was inflated, saying he had found instances where it appeared schools had wrongly changed student grades and improperly awarded credit.

But Mr. Thompson, a candidate for mayor, did not point to any conclusive evidence of manipulation, saying only that a lack of oversight, coupled with intense pressure to push up the graduation rate, created the potential for abuse. And he acknowledged that, by and large, the schools examined in his report were awarding diplomas only to students who had met graduation requirements.

Still, Mr. Thompson cautioned, the city was not monitoring students’ records scrupulously enough and the record-keeping system was disorganized and prone to inaccuracy.

“The mayor’s managerial style has created an incentive for schools to graduate students whether or not they have met the requirements,” Mr. Thompson said at a news conference. “A New York City high school diploma is supposed to represent one standard.”

In a 38-page rebuttal, the city’s Department of Education defended its graduation rate, which has climbed to the highest level in decades (56 percent in 2008, up 10 percentage points since 2005).

Jennifer Bell-Ellwanger, the education official who wrote the rebuttal, said Mr. Thompson had misinterpreted how schools maintain records.

“The comptroller focuses his report almost exclusively on minor quibbles,” Ms. Bell-Ellwanger wrote, “and raising unwarranted, unfair suspicions regarding school conduct, based on cursory and incomplete inquiry.”

In an interview on Tuesday evening, Mr. Thompson also called for the firing of the schools chancellor, Joel I. Klein, in part because of the findings in the report. He said Mr. Klein, a lawyer, had failed to run the Education Department in a transparent manner and had shut parents out of the system.

“I think it is time to go back to a chancellor who is an educator,” he said.

Mr. Thompson, a Democrat, has been criticized for running a subdued campaign in his effort to unseat Mayor Michael R. Bloomberg.

The mayor often trumpets increased test scores and the graduation rate as a hallmark of his tenure, and he has made his education accomplishments a focal point of his re-election campaign.

Mr. Thompson appears to be trying to undermine that talking point, issuing a string of news releases questioning the progress in the schools during Mr. Bloomberg’s watch.

The graduation rate report released on Tuesday was no exception. A news release from the comptroller’s office said it was “blistering audit,” asking, in capital letters: “Did graduates actually meet all requirements to earn their diplomas?” At the news conference, Mr. Thompson called the city’s school system the “Enron of American education.”

Mr. Bloomberg’s re-election campaign quickly responded, issuing a statement attacking Mr. Thompson’s record as president of the city’s Board of Education in the 1990s and saying the report was based on “phony attacks.”

Mr. Thompson’s yearlong audit of 197 students from the class of 2007 found insufficient evidence to verify that 19 of the students had met requirements for graduation. Education officials eventually provided enough documentation to convince Mr. Thompson that at least 17 of those students had met the requirements, but the comptroller said he had no indication that schools had assembled the information before awarding diplomas.

In addition, Mr. Thompson’s report found that 18 percent of the students in the study were given credit for repeating courses they had passed. Department of Education officials disputed that claim. In one instance, Mr. Thompson, said a student was twice given credit for English 4, but education officials said the student had taken one of those courses at summer school, and that the course content had been different.

Mr. Thompson also argued that 25 percent of student transcripts he studied were altered before or after graduation, and that educators were not submitting proper grade-change forms. But education officials said changes to transcripts were part of regular reviews of student data at the end of the year as school officials entered the information and found errors.

Mr. Thompson also took aim at the city’s dropout rate, which he said was artificially low. He said the city had overstated the number of students who had transferred to private schools or moved from the city, known as “discharges.” Mr. Thompson accepted the city’s explanation for 13 of the 14 cases that he challenged, but said he was concerned education officials had classified them as discharges without proper documentation.

Mr. Thompson called on the city to better monitor student transcripts and institute citywide policies on how course credits are earned. Education officials said that their practices were already in compliance with state law, and that the comptroller’s demands exceeded those requirements.

Education officials also said there was no evidence that schools had made decisions to graduate any students without consulting the proper documents.

To graduate, students must earn 44 credits and pass Regents exams.

A frequent criticism is the city’s use of credit makeup programs, which allow students to complete packets of worksheets or enroll in weeklong cram sessions over holiday breaks to earn credit for courses they have failed.

Some educators say the programs have eroded the rigor of academic programs and allowed students to accumulate credits when they have not mastered the material. But the city noted that graduation requires more than credits, and that any such abuse is not widespread enough to affect the graduation rate.

(LINK to NY Times article)