During the debate, John Kasich, Donald Trump, and Ted Cruz were asked about Common Core State Standards. Of the three, Trump barely answered the question (albeit without the specifics Jake Tapper called for), Kasich gave the wimpy "it's all hysteria" answer, and Cruz actually identified the problem.
Common Core's biggest problem is that the U.S. Department of Education has been using other programs like Race To The Top funding in order to more or less blackmail states into adopting the standards. There are very, very few curricula written to the standards, and the majority of the ones out there are barely comprehensible. Cruz's plan to roll back the executive actions that have led to the abuse of education funding and eventually eliminate the Department of Education is the most sensible way to go about de-federalizing Common Core.
Good on Cruz for knowing the problem, when it's clear the other two have no idea.