"Consistently, the New Jersey Education Association has shown it is unwilling to accept reforms that put results for our children first and use them as part of our metrics for evaluating teacher performance," Hendricks wrote in the e-mail, which the Department of Education released.
"Over and over," she added, "the NJEA has only been interested in protecting the status quo that continues to fail students, particularly in our urban schools."
The state's schools "are led by some of the finest, most talented teachers in the nation," Hendricks said. But "we need fundamental change, and the need is urgent."
Keshishian called Hendricks' comments "an insult to every teacher in New Jersey" and defended the union's reform record on Wednesday.
"Once again, the administration thwarts any attempt on our part at cooperation and collaboration because of the governor's larger political agenda that insists that teachers and their union are enemies of public education," Keshishian said in a statement.
It is a decades-long tradition for education commissioners to participate in a question-and-answer session with teachers at the convention, according to NJEA spokesman Steve Wollmer.
"The commissioners always come," Wollmer said. "I think once, the commissioner was sick and couldn't come."
Tens of thousands of education professionals attend the annual event, according to the NJEA.
Christie spokesman Michael Drewniak said the e-mail was Hendricks' but added that it "reflects our thinking" on the NJEA and reform.
Hendricks' planned no-show is the latest volley in a bitter war between the administration and the union.
Christie wants to institute merit pay and make changes to the tenure process, which the NJEA opposes. Last week, Christie named a nine-member task force to explore evaluating public-school teachers by their students' performance, which the NJEA does not consider educationally sound. No NJEA representatives were named to the panel.
Contact staff writer Rita Giordano at 856-779-3841 or firstname.lastname@example.org.