Mayor Rahm Emanuel once praised the Noble network of charter schools in Chicago as having a “secret sauce” for success. Part of its “secret sauce” was fining students $5 for every disciplinary infraction. Some families owed the school hundreds or even thousands of dollars. That was one way of pushing out difficult students. It works. But that’s not all.

A new report from Chicago Public Schools shows that one important ingredient of the secret sauce is expulsions. It makes perfect sense. If a school can kick out the kids with low scores, the school will have higher scores and the public school that gets the low-scoring kids will have lower scores. How simple! Charter advocates like to say that public schools should copy the successful strategies of the charter schools. Now, if the Chicago public schools learn from the charters, who will take those kids? Ideas?

As it continues to modify strict disciplinary policies in an effort to keep students in the classroom, Chicago Public Schools on Tuesday released data showing privately run charter schools expel students at a vastly higher rate than the rest of the district.

The data reveal that during the last school year, 307 students were kicked out of charter schools, which have a total enrollment of about 50,000. In district-run schools, there were 182 kids expelled out of a student body of more than 353,000.

That means charters expelled 61 of every 10,000 students while the district-run schools expelled just 5 of every 10,000 students.

It’s the first time the district has released student suspension data for every school and also the first time it has released data on expulsions for charters. For charter critics, the numbers will buttress long-standing complaints that the privately run operations push out troubled students, allowing their schools to record stronger academic performances.