In 2001, the third-longest serving senator supported the No Child Left Behind Act, which was first proposed by then-President George W. Bush. The bipartisan bill made it a requirement for states that received federal funding to develop a standardized assessment test for students in certain grades.
Supporters of the bipartisan bill said it increased accountability, but critics argued against standardized testing, saying it is not a true test of a student's ability.
"This is a defining issue about the future of our nation and about the future of democracy, the future of liberty, and the future of the United States in leading the free world," Kennedy said of the bill on the Senate floor. "No piece of legislation will have a greater impact or influence on that."
Kennedy's support earned him a fair share of controversy, even within his own party.
"I want you to think about this, and I have to say, this was a train wreck that was not intended. No Child Left Behind was supported by George Bush and Sen. Ted Kennedy and everybody in between. Why? Because they didn't talk to enough teachers before they did that," Bill Clinton said of the act in 2008.
But despite the criticisms, some say Kennedy is likely to be remembered as one of the greatest Senators of his time.
"I think they're gonna say he is one of the greatest legislators, or most effective legislators -- if not the most effective legislator -- the Senate has ever seen," Boston Globe reporter and author Susan Milligan said. "And I don't think you could find a sitting senator right now, Democrat or Republican, who would disagree with that assessment."