There has been much debate about a longer school days in Chicago Public Schools. But a comparison between Chicago and suburban schools made by the Chicago Tribune hints at something: the length of the school day is not the key determinant of student outcomes.

The tongue-lashings Chicago Public Schools has endured in the last several weeks over its short school day — U.S. Education Secretary Arne Duncan called it a “disgrace” — have overshadowed the fact that that many suburban students aren’t receiving much more instruction time than CPS.

Affluent Glen Ellyn’s two elementary districts both offer five hours, 15 minutes of instruction daily, only seven minutes more than CPS reports…

With state data unreliable, the Tribune used class schedules from a handful of Chicago-area districts to highlight some of the discrepancies. So while seventh-graders in northwest suburban Elgin School District U-46 are getting less than five hours, 30 minutes of instruction on average, their counterparts in southwest suburban Plainfield District 202 are receiving about seven hours, according to state records.

That’s a big difference, but one that doesn’t necessarily translate into student performance, experts say. Indeed, at a time when urban and suburban districts across the U.S. are lengthening their school days in an effort to improve tests scores and student learning, no studies conclusively link more instruction time with higher achievement.

I can think of several reasons why there has been so much attention on the length of the school day in Chicago:

1. This seems like common sense: kids will learn more if they are in school longer. However, studies suggest it is more about how time is used rather than just have larger quantities of time. And if more time was really needed, why not have a serious conversation about shorter summer breaks and possible Saturday programs?

2. It is part of a larger back and forth with teachers. Thus far, the union has not been willing to lengthen the school day and Mayor Emanuel and his team has tried to split teachers on their stance. This is not the only source of disagreement between the District/the mayor and the teacher’s union but it has been very public.

3. The school day is one of the few things that the District can more easily control. Compared to other possible solutions like improving the skills of teachers or hiring better teachers, helping improve life in poorer neighborhoods, or getting parent’s involved, this looks like an easy target.

Next year, the Chicago Public Schools will have a longer school day in 2012-2013. While leaders may take credit for this, it will be interesting to see if there is any positive outcome (and then it is another question about whether this is due to the longer school day). Additionally, if they just stop at longer school days, not much will have changed.

This reminds me of the Coleman Report which had a few findings: “student background and socioeconomic status are much more important in determining educational outcomes than are measured differences in school resources (i.e. per pupil spending)” and “socially disadvantaged black students profited from schooling in racially-mixed classrooms.” But getting school districts and the general public to get ahead these ideas (think of the debate over busing in the late 1960s and early 1970s) is a very difficult task.