The year-round calendar affects about 90,000 students in the nation's third-largest district.
Chicago Public Schools CEO Ron Huberman reminded parents and students to stay informed, especially with it being a new calendar for many.
The year-round calendar has the same number of school days as a traditional school year, but the year is broken up with several breaks throughout the year.
About 408,000 students attend Chicago Public Schools.
The Amelia Earhart School on East 93rd in the Calumet Heights neighborhood on the South Side is now one of the year-round schools.
The school year involves eight weeks on, two weeks off, all year round.
"The breaks are shorter and spread out more. Students retain more and do better," said Huberman.
Some contend it is a safer system.
"It gives our kids a safe haven. For kids who don't have a place to go or something to do during the summer, this provides them an opportunity to be in a safe environment," said Ald. Michelle Harris, 8th Ward, who opened the Earhart school by ringing the first bell Monday.
There had been opposition from some because it disrupts some families' schedules.
There have also been improvements to many of the year-round schools to beat the summer heat while students are in class.
"On the new wing, we have central air. And all of the classrooms have air conditioning. So the children will be comfortable in their classrooms," said Dr. Brenda DeMar-Williams, Earhart principal.
For those not going year round, the traditional school year will start September 8.
The Associated Press contributed to this report.