The way we assess school quality is changing fast. When Chicago last ranked public schools, in 2012, we followed standard practice at the time and threw a lot of weight on test scores. For one thing, they were a widely available and consistently reported measure of performance. For another, standardized test results—typically expressed as a percentage of students who meet or exceed state standards on a given exam—offered nice, clean, ostensibly meaningful numbers that could be compared at a glance.
But as any teacher or principal will tell you, test scores aren’t everything. They may be useful for showing academic achievement at a single point in time, but they don’t show gains or losses over the duration of students’ tenure. And with Chicago’s disparate student population, that can be a big limitation, because students in poor neighborhoods tend to enter the system at lower levels than students in well-off neighborhoods or those who test into selective enrollment institutions.