Class sizes across the city could increase by an average of 13 percent next year if the city decides to cut more than 6,000 teaching positions, according to a report the teachers union released today.

The United Federation of Teachers’ report doesn’t consider whether the city will lay off teachers by seniority — as is currently mandated by law. Nor does it factor in Mayor Bloomberg’s desire to lay off teachers based on how their principals have rated them, or how many unexcused absences they’ve accumulated, among other factors. Instead, it takes a blunt measurement of what the loss of over 6,000 bodies in classrooms could do to class sizes across the city and in certain districts.

The calculations don’t appear to take into account many of the complicated details behind how schools distribute their teachers. Often, schools will keep class sizes low for younger students, then increase them for older ones. Schools that separate their advanced students from those who are struggling are also likely to keep class sizes high for the former and low for the latter.

Figuring that 6,000 teachers — 4,666 lost through layoffs, 1,500 through attrition — make up roughly 11 percent of the general education teaching force, the union’s report begins by assuming an 11 percent teacher loss equates to an 11 percent  class size increase. Its researchers then added a two percent additional increase to arrive at 13 percent. The report states:

It makes sense to add another 2 points to that 11 percent. Over the last three years, class sizes have risen an average of about 2 percent a year, as the number of teachers has declined, funding for class size reduction has been frozen and enrollments have unexpectedly increased.

Assuming that job losses would affect all grade levels equally, the reports states that the average class size in kindergarten through third grade could rise from 23.1 students to 26.1. In grades 6-8 it could increase to as much as 30 students in a room. Under the current teachers union contract, no kindergarten class can exceed 25 students and no class in grades 1-6 can top 32 students.

The union’s predictions exclude special education students and their teachers because these class limits are set by law. Department of Education officials have yet to respond to the union’s report, but I’ll update the post when they do.