Across the city, classes this year are larger, on average, than they were last year, according to data the Department of Education released today.

The new data, released this afternoon to meet an annual reporting deadline set by the City Council, show that class sizes have increased citywide for the sixth year in a row, with the largest increases coming in high schools.

Overall, class sizes jumped by an average of 1.6 percent this year. Classes in elementary schools now average 24.5 students; middle schools average 27.3 students per class; and high schools have 26.9 students on average in each class.

In September, a tally by the teachers union found that 670 schools — more than ever — had classes over their contractual size limits, which are higher than the citywide class size averages.

“Every parent and every teacher knows how critical it is that classes are small enough that each child can get individual attention,” UFT President Michael Mulgrew said in a statement responding to the new data. “But Mayor Bloomberg disagrees.”

Bloomberg began his tenure as mayor pledging to reduce class size, but now he says he doesn’t see class size as a pressing issue and would opt for better teachers over more teachers. A year ago, he said during a speech in Massachusetts that if he had his druthers, he would fire half of the city’s teachers and use the other half to lead larger classes. “Double class size with a better teacher is a good deal for the students,” Bloomberg said.

The changes reported today were, unsurprisingly, far less dramatic, with classes growing by .4 students on average.

Still, classes in kindergarten through third grade, where research has most clearly shown that smaller classes are better for student achievement, are now only slightly smaller than they were in the 1998-1999 school year, when they had 24.9 students on average. Last year, classes in those grades averaged 23.9 students; this year, that figure was 24.5.

Classes grew by the biggest margin this year in high schools, where classes grew by .6 students on average, from 26.3 to 26.9 students. The department’s PowerPoint presentation about the data suggested that a policy change requiring all high school seniors to take a full schedule of classes had fueled the increase.

In addition to the presentation, the department published detailed spreadsheets showing class sizes in each school and across different kinds of classes, such as type of special education class or high school subject.

The report released today is the first of two about class size the department is required to put out each school year. The department proposed eliminating the first report and completing only one in February instead, a move that advocates of smaller class sizes charged would produced artificially low class sizes. A city advisory board has recommended keeping both reports.

This year’s report was based on official enrollment data on Oct. 26, the last day of school before Hurricane Sandy hit the city. Some storm-affected schools have seen their enrollments change dramatically since then.