One of the big, undecided policy issues of the post-Obama Democratic Party is whether education reformers or teachers unions and their allies will control the party's education policy. Hillary Clinton said something over the weekend that made unions very happy and reformers very worried.

"Charter schools can have a purpose, but there are good charter schools and there are bad charter schools, just like there are good public schools and there are bad public schools," she said at a Congressional Black Caucus event in South Carolina:

The original idea behind the charter schools… was to learn what worked and apply them in the public schools. And here's a couple of problems. Most charter schools, I don't want to say every one, but most charter schools, they don't take the hardest-to-teach kids. And if they do, they don't keep them. The public schools are often in a no-win situation, because they do thankfully take everybody, and they don't get the resources and help and support they need to take care of every child's education. I want parents to be able to exercise choice within the public school system. Not outside of it, but within it. I am still a firm believer that the public school system is one of the real pillars of our democracy, and it is a path for opportunity. But I am also fully aware there are a lot of substandard public schools. But part of the reason for that is policymakers and local politicians will not fund schools in poor areas that take care of poor children to the level they need to be.

This is a very different tone than you'd hear from the charter-friendly Obama administration.

Nationally, charter schools students perform about as well as public school students. In most cities, charter schools educate a smaller proportion of kids with disabilities and kids learning English than public schools in those cities. It's much more tenuous to argue that "most" charter schools deliberately push out kids who are hard to teach, although, as the New York Times recently reported, a branch of the wildly high-achieving Success Academy charter school chain kept a list of students who had "got to go."

But Clinton's remarks — which were in response to a charter-friendly question that noted black parents are heavily in favor of such schools — signaled whose side she was on in the contentious Democratic Party education debate.

On education, 2016 is not 1996

Teachers unions and other groups skeptical of reform make the same argument Clinton is advancing here: that charter schools have an unfair advantage due to the students they enroll, that charter schools should enrich public schools rather than competing with them, and that the big problem for public schools is poverty and lack of resources.

Clinton and her husband were early supporters of charter schools, which were a brand new idea when they took office. Some education reformers still hope this means she'll continue to support them.

But in the 1990s, charter schools were a Clintonian triangulation. The hot conservative education idea at the time was school vouchers — giving students public money to attend private schools. Charter schools, which are publicly funded but privately run, were a way of offering some choice and flexibility to families without giving up entirely on the notion of publicly funded K-12 education.

Since then, though, vouchers have largely dropped out of the national conversation (although they could still make a comeback among Republicans in 2016), while the Democratic Party has divided between education reformers like Arne Duncan and teachers unions, their traditional backer. The unions have endorsed Clinton, and the evidence so far is piling up that her vision really does align more closely with theirs.

Charter schools suspend students at a high rate

Urban charter schools are generally better than traditional public schools at increasing students' reading and math scores. They also discipline their students far more frequently, and both critics and some supporters argue this partly explains why they're more successful.

Charter schools in New York suspended students about three times as frequently as public schools in 2011-'12, the last year for which data is publicly available. The same year, charter schools in DC expelled 72 out of every 10,000 students, according to a Washington Post investigation — 72 times the expulsion rate of traditional public schools. (They've since reduced the rate to 34 out of 10,000, a major decrease, although still greater than the traditional public school system.) In Chicago, charter schools expel students at 13 times the rate of traditional public schools.

The higher suspension and expulsion rates don't count students who leave voluntarily. This forms part of a larger critique of charter schools: They're able to manage their student body so that they have students who are better prepared to learn.

Studies of high-performing charter schools are able to account for demographic issues and nonetheless show a charter advantage. A detailed evaluation of KIPP charter schools, a highly regarded urban charter school chain, found that students changed schools at about the same rate at district schools and charter schools. Students' departures accounted for, at most, one-third of KIPP's advantage on standardized tests.

Still, some charter school leaders argue that tight discipline is key to their success, not by selecting out low-performing students but by creating an environment in which most students can thrive. If that's true, charters have an advantage traditional public schools don't.

Charter schools can expel students, or suspend them so frequently that their parents decide to send them elsewhere, in part because district schools usually exist as a backstop — as Clinton says, "they thankfully take everybody." That doesn't mean that test scores are high only because low-achieving students are pushed out. But it does suggest a symbiotic relationship between charter and traditional public schools that charter backers don't always acknowledge.