I don't doubt that, but again, when lower performers are dropping like flies, they don't show up in the stats. Public schools keep all the lower performers, so their results naturally look worse. Indeed, they also take all the kids who are pushed out of Urban Prep.
The question that needs to be asked is just how much of that extra 8% is the result of actual education, and how much is the result of stat fluffing.
We should also recognize that spending an extra 1/3 and several more hours per student starting in 9th grade shouldn't be producing such low gains for 12th graders in terms of these exams. They are still far below average, including those of public school students with less money and resources outside of those neighborhoods.