There are a couple reasons for this. First, graduation decisions are mostly left up to the schools and districts that are supposedly accountable for them. If you hold schools and districts accountable for their graduation rates, they have an incentive to just pass more students along and out the door.
Second, graduation rates also have a long time lag between when a student begins his or her high school career and when he or she finally finishes. Although schools and districts have quite a bit of control over these things, there’s still a long time lag, and it will be hard for a superintendent or principal to show any immediate improvements.
Both of these issues can be mitigated in various ways. States adopted high school graduation exams to require all students across the state to meet the same bar. But those exams come with their own problems. To avoid the time lag problem, states could adopt retention measures calculated on an annual basis. But few states have done so. Averaging across multiple years could also help, but few states do that either, and schools still can’t move the needle very quickly.
But worst of all, graduation rates don’t tell us very much about whether students are prepared for life after graduation.
The graph below comes from my new paper on high school accountability systems. The graph plots graduation rates against college-going rates for each high school in Tennessee. Each dot represents one school, and the solid diagonal line represents the relationship between the two variables. As the line suggests, there is a trend that schools with high graduation rates also have high college-going rates.
But the general trend misses a lot of variation. Schools with nearly identical graduation rates can differ by 50 or 60 points in terms of the percentage of graduates who go on to college (compare all the dots in the red circle).
If you construct an accountability system based purely off graduation rates, you’re going to get some perverse results. Using the same data on Tennessee’s high schools, I created lists of “low-performing schools” based on how the schools ranked on different measures of school quality. Of the schools with the lowest graduation rates, none were also in bottom tier on student growth, and only six percent were in the group of schools with low college-going rates.