Nearly half of the city's school children attend classes in buildings that are over capacity, according to data from the city's Independent Budget Office released Tuesday.

During the 2012 to 2013 year, 43.5% of students were being taught in overcrowded buildings. Nearly 600 school buildings, or 41.2% of them, were deemed overcrowded, up from 39.5% the year before. The average K-8 class size increased to 25.5 children from 24.6 children during the 2010-2011 year.

Mayor Bill de Blasio said the overcrowding issue will be addressed with new buildings funded from a "very substantial capital commitment in this budget," though he admitted there are still pockets with overcrowded facilities, particularly in central Queens.

"But you'll see a lot of new construction activity over the next few years to address that," de Blasio said. "You're also going to see a movement away from trailers wherever we can do that, as quickly as we can do that. The capital money will help achieve that."

The number of new school seats and buildings has dropped off in recent years, according to the IBO report. During the last school year, 9,037 new seats were added, which was down from the nearly 13,000 new seats created in the 2009-10 school year.