Hillary & Bernie Gagged on K-12 Education Policy Discussions by Unions?

That's the gist of this piece by Mark Hemingway in the Weekly Standard. He writes,
In August, [Campbell] Brown organized an education reform forum in New Hampshire with six contenders for the GOP presidential nomination. It was a wide-ranging and informative discussion in a primary season where substantive policy discussions have been largely, uh, trumped. Brown was all set to hold a similar event with Democrats in Iowa on October 22 that was to have been cosponsored by the Des Moines Register. But the order came down from on high that Democrats were not to participate, and the forum was called off. 
“What happened here is very clear: The teachers’ unions have gotten to these candidates,” Brown told Politico. “All we asked was that these candidates explain their vision for public education and how we address the inequality that leaves so many poor children behind. .  .  . President Obama certainly never cowered to the unions.”
No doubt therein lies a pretty fat kernel of truth. And yet, is it too much to ask from lifelong Democrats (like me) that our Party's presidential contenders float a few hints about where they stand on accountability, school choice, and higher standards? I mean, besides Clinton's recent blooper (see FactCheck.org) about charter schools?

Look, Bernie and Hillary: campaign pablum about preschool and escalating college costs don't count as meaningful insights into your respective education agendae. Give us a glimpse, please. If you're not being muzzled, then talk. If you're care about K-12, then speak.