Ezra Klein:

Chicago performs quite poorly on national assessments of educational quality. As Reuters notes, fourth-graders in Chicago performed an average of nine points worse than the big city average and sixteen points worse than the national average on the math section of the National Assessment of Educational Progress (NAEP), the national gold standard for measuring learning. On reading, they were eight and seventeen points worse than big city and national averages, respectively. That’s a bit better than Los Angeles or Washington, D.C. but worse than New York.
Chicago also has shorter than average school years and school days. Many students are only in class for 170 days a year as of a few years ago, below the state minimum of 176 days and the national average of 180 days; under Emanuel, the year was lengthened to 180 days. The school day in Chicago averages five hours and forty five minutes in elementary schools (as opposed to the national average of six hours and forty-two minutes) and seven hours for secondary schools, above the national average of 6.6 hours. Emanuel and teachers recently negotiated a deal to hire 500 new teachers to allow for a 90 minute school day extension without increasing hours for current teachers.