Gerald Conti retired from teaching after he worked for 27 years at Westhill High School in New York.
Conti, 62, wrote the critical letter and detailed what has happened to the profession over the years. He posted it onto Facebook in March and it has since been shared thousands of times.
In it, he said lawmakers "failed us by selling children out to private industries such as Pearson Education."
He also said the New York State United Teachers union failed by not trying to fight standardized testing, resulting in "a pervasive atmosphere of distrust" which prevented instructors from coming up with their own methods of evaluation.
"After writing all of this I realize that I am not leaving my profession, in truth, it has left me. It no longer exists," he said.
Once it had almost 1,000 shares on Facebook, Syracuse Post-Standard picked it up and published it.
Conti told the Post-Standard that he did not intend for the letter to be critical of his district, but to be critical of education as a whole.
"This whole thing is being driven by people who know nothing about education," he said. "It's sad."
His letter comes after many teachers have criticized the system.
In Seattle, teachers refused to give a Measures of Academic Progress test this year on "ethical and professional grounds." In Texas, a rally of 10,000 showed their disdain for standardized testing.
"Education to me is completely qualitative, it's not quantitative," Conti said. "It's about personal relationships, it's about getting kids to be curious. And that's what I've been trying to do all my career."