In a reversal, Gov. Andrew Cuomo vetoed a bill Monday night that would have protected teachers from getting bad ratings if their students did poorly on the state's tests.

Cuomo had drafted the "safety net" bill last spring in response to concerns from teachers unions who worried their students needed more time to adjust to tests aligned with the new Common Core standards. The percentage of students scoring proficient on state exams dropped precipitously after the tests changed in 2012. But Cuomo rejected the need for any safety net legislation after just 1 percent of teachers received low marks in ratings released this month.

Teacher evaluations that include student test scores and observations by supervisors went into effect statewide in 2013. In a year-end report highlighting his accomplishments, Cuomo said they represented a "sea change." But he went on to say this "lays the groundwork for the state to improve the evaluation system so that it is a rigorous, accurate and reliable measure that will allow districts to meaningfully differentiate between educator."

Cuomo had already sent a letter to state education officials saying he wanted to pursue a more aggressive reform agenda in 2015.

New York State United Teachers complained that the governor's reversal isn't fair because teachers are still held accountable for scores on the new Common Core tests, even though new legislation protects students who get low marks when they apply to other schools. They also can't be held back a grade based solely on test scores.

"The governor reneged on an agreement," the union said in a statement.  "With this veto, the governor has decided that teachers are the only ones who should be held accountable for the state's failed implementation of the Common Core.  We can't understand why he is refusing to sign his own bill.  What has changed?"

The union accused the governor of "doing the bidding of billionaire hedge fund managers" who criticize unions while supporting privately run charter schools.

Cuomo's office did not respond to a request for comment.