Today is the first day of class for at least 90,000 students at Chicago Public Schools who are switching to a so-called year-round calendar.

The students' 2009-10 school year begins today rather than after Labor Day, when Chicago students typically return to class.

In April, the district increased the number of schools that operate on the year-round calendar, known as Track E, from 65 to 132. The CPS system has more than 600 schools in all.

School officials, among them CPS Chief Executive Officer Ron Huberman, have promoted the year-round calendar as a way to minimize learning loss during the summer, improve lesson plans and curb teacher burnout. The calendar change also takes children off the streets for more of the summer, which supporters say is a plus in neighborhoods with high crime, gangs and drugs.

The traditional school calendar has a 10-week summer break. Students enrolled in the year-round program have the same number of school days, but they take shorter, more frequent breaks. They take the same holidays off and end school on the same day as students on the traditional schedule.

According to district officials, principals must request a change to the year-round program after consultation with staff and community members. Records show it is a request they have made with increasing frequency in recent years.

In August of 2008, the number of year-round schools rose more than twofold from 18 to 41. In March, the Board of Education added 24 additional schools to the roster. The April additions comprised the largest group of schools to switch to the year-round calendar yet.