The number of failing city schools increased by 12 this school year, the State Education Department has announced, reversing a downward trend in the number of failing schools for the first time in three years.

In total, 421 schools, or about a third of city schools, were on the department’s failing list. Three hundred thirty-five city schools that receive federal anti-poverty funding were listed in need of improvement under No Child Left Behind requirements this school year, compared with 332 last school year, with 45 high-poverty schools added to the list for the first time. The number of lowpoverty schools on the list also rose, to 86 from 77.

The majority of schools on the list are elementary schools, which make up the largest percentage of city schools. Students attending a high-poverty school on the improvement list can request a transfer under the federal law.

The state usually announces its list of failing schools in September. Though 11 new failing high schools were announced then, the list of elementary and middle schools was delayed because 2006 test results came back late, state officials said. State tests were given to third- through eighth-graders for the first time last school year, instead of fourth- and eighth-graders exclusively, as in years past.

City Department of Education officials blamed the increase in failing schools in part on the larger number of students being tested.

“We are pleased that the percentage of New York City schools in good academic standing held steady in a year when three times as many students were held to state accountability standards,” the schools chancellor, Joel Klein, said in a statement. “Today’s announcement shows that we have made lasting improvements in our schools and need to continue to make progress in order to ensure success for all our students.”

Mr. Klein congratulated 39 schools — including 27 high-poverty schools and 12 low-poverty schools — that were removed from the list this year.

The president of the United Federation of Teachers, Randi Weingarten, suggested in a statement that the rise in the number of failing schools could fuel further bureaucratic restructuring by the Department of Education. She also criticized the federal formula used to determine a school’s progress, which samples subgroups of students at each school. “As we have long predicted, based on the current federal NCLB Adequate Yearly Progress formula, we will soon see all schools deemed to be in need of improvement,” she said.

High-poverty schools that do not meet tough federal standards known as “adequate yearly progress” for two years are added to the failing list. To get off the list, schools must make adequate yearly progress for two years in a row. Schools that are on the list for more than two years must make a plan to restructure, which can include replacing teaching staff or the principal, changing the curriculum, or closing. After their fourth year on the improvement list, schools must put the restructuring plan into action.

Students who attend high-poverty schools that have been on the list for two years can obtain outside tutoring from private organizations that are funded by the government.