New data released by the Chicago Public School system reveals that the city’s public charter schools — whose expansion has been pushed by Mayor Rahm Emanuel for several years — expelled about 12 times the number of students last year than did traditional public schools.

The Chicago Tribune reported that in 2013, charter schools with a total enrollment of about 50,000 students expelled 307, while the traditional school system, with more than 353,000 students, tossed out 182 students. That, the Tribune said, meant that “charters expelled 61 of every 10,000 students while the district-run schools expelled just 5 of every 10,000 students.”

The high charter school expulsion rate is not singular to Chicago; such practices have been documented in many other cities where charter schools have grown as well. My colleague Emma Brown wrote in this January 2013 story that

D.C. charter schools expelled 676 students in the past three years, while the city’s traditional public schools expelled 24, according to a Washington Post review of school data. During the 2011-12 school year, when charters enrolled 41 percent of the city’s students, they removed 227 children for discipline violations and had an expulsion rate of 72 per 10,000 students; the District school system removed three and had an expulsion rate of less than 1 per 10,000 students.

What these statistics do is underscore the unfairness of comparing charter schools and traditional public schools in terms of student achievement. Why? Many (though not all) charter schools are very aggressive in separating the school from students who are seen as especially difficult to educate. Traditional public schools don’t have the same luxury; they have to take all students — including the ones thrown out by charter schools — and have a much harder time expelling them.

This, of course, affects school-wide standardized test score averages, which are the chief metric by which schools and districts are being evaluated in this test-obsessed school reform era. Even if you take charter advocates at their word that charter schools don’t toss out kids to improve their academic record (and in some cases that is known not to be true), the fact is that claims that charter schools are educating the same population of children as traditional schools are simply not true in many cases.

This was the first time this data was released in Chicago, and the district’s chief executive officer, Barbara Byrd-Bennett, acknowledged that the statistics will be controversial. The Tribune quoted her as saying:

“I think there’s been a lot of supposition and conversation about what and how the charter success is measured, whether they throw kids out or they keep kids in. I think having the data is going to now lead to productive conversations.”

For there to be productive discussions in Chicago, Emanuel will have to face some facts. On her blog, education historian and activist Diane Ravitch noted that Emanuel “once praised the Noble network of charter schools in Chicago” for having a “secret sauce” for success, and part of it was fining students $5 for every disciplinary infraction — leaving some families owing thousands of dollars to the school. According to the Tribune, the three campuses in the Noble network expelled between 2 percent and nearly 5 percent of their students in the last school year. That sounds like it could be a “secret sauce” ingredient, too.