Who is doing the opting out?

The New York Times ran a story earlier this week about opting out of standardized testing, widely blaming/crediting (depends on your perspective) teachers' unions with fueling the trend:

After several years in which the teachers' unions have been hammered on the issue of tenure, have lost collective bargaining rights in some states and have seen their evaluations increasingly tied to student scores, they have begun, with some success, to reassert themselves using a bread-and-butter issue: the annual tests given to elementary and middle school students in every state.

I was amazed by the laziness of this reporting, honestly, right down to the pro-forma quote by Jonah Edelman from Stand for Children.  No mention of United Opt Out, not a single quote from a parent, no conversations with students...it really reads like phoning it in.
This did lead to some interesting responses, including Andy Smarick's about how ed reform responds to opting out; he urges his compatriots not to repeat their mistake of dismissing critics as they've done with the Common Core.
Secretary Duncan, at the Education Writers of America, commented that opting out may require federal action:

On Tuesday, when asked whether states with many test boycotters would face consequences, Duncan said he expected states to make sure districts get enough students take the tests.
“We think most states will do that,” Duncan said during a discussion at the Education Writers Association conference in Chicago. “If states don’t do that, then we have an obligation to step in.” 

 I will leave it to others to debate the relative constitutionality of that. I will, however, point out that pulling federal funds, which is about the only consequence open to the federal government, will only continue to widen the rich/poor divide, as it is poorer districts that receive the majority of federal funds.
Possibly related, Senator Tester of Montana filed an amendment to the ESEA renewal that would strike the requirement for grade testing.
Finally, Bruce Baker points out that massive opt out has already happened by parents who don't send their kids to public school, thus all comparisons are moot already.