A federal study showing that fourth graders in charter schools score worse in reading and math than their public school counterparts should cause some soul-searching in Congress. Too many lawmakers seem to believe that the only thing wrong with American education is the public school system, and that converting lagging schools to charter schools would cause them to magically improve.

The study, based on data from 2003 on students’ performance on the National Assessment of Educational Progress, found charter school students significantly behind their non-charter-school counterparts. But it also showed that not all charter schools are created equal.

On average, charter schools that were affiliated with public school districts performed just as well as traditional public schools. That may be a disappointment to advocates who expected them to show clear superiority. But the real stunner was the performance of free-standing charter schools, which have no affiliation with public school systems and are often school districts unto themselves. It was this grouping that showed the worst performance.

Free-standing charter schools often bite off more than they can chew. The presumption is that without the bureaucratic restraints of the public school system and the teacher unions, charter schools can provide better education at lower cost. But the problem with failing public schools is that they often lack both resources and skilled, experienced teachers. While there are obvious exceptions, some charter schools embark on a path that simply recreates the failures of the schools they were developed to replace.