The number of schools receiving A’s under New York’s much-contested grading system increased significantly this year from last in what Mayor Michael R. Bloomberg said was a clear sign of success and evidence that his signature accountability program was spurring improvement at schools across the city.

Nine of the schools that got F’s for the 2006-7 school year got A’s in this year’s grading; just one of last year’s A’s plummeted to an F this time around. Critics immediately seized on the wild fluctuations as proof that the system was flawed because of its emphasis on year-to-year progress by individual students rather than multiyear gains.

Over all, nearly 80 percent of the 1,040 elementary and middle schools judged got A’s or B’s, while the number labeled failures dropped to 18 from 43. The percentage of schools getting A’s jumped to 38 percent, from 23 percent last year. About 57 percent of last year’s A schools received A’s again, while no school was deemed an F twice.

“Not a single school failed again,” Mr. Bloomberg said at a news conference at Public School 5 in Bedford-Stuyvesant, one of the F-to-A schools. “That’s exactly the reason to have grades, to show what you haven’t done yet and what you do to improve. The fact of the matter is it’s working.”