NEW YORK (MainStreet) — The resignation of Education Secretary Arne Duncan, which was announced on Friday afternoon, was unexpected but perhaps overdue. One of the longest serving members of President Obama's cabinet, Duncan has been a disappointment for many progressives within the Democrat party who looked forward to a roll back of Bush administration initiatives such as No Child Left Behind, which focused on standardized testing rather than critical thinking skills.

In a letter to his staff, Duncan said he's going back to Chicago to be with his family. He disclosed no immediate plans, but said anything he's likely to do will "continue to involve the work of expanding opportunity for children." He will stay on at ED until December.

Duncan became a principal cheerleader for the Common Core State Standards, a national test-everything approach to student and teacher evaluation that grew out of the 1990s American Diploma Project and the standards and accountability movement. Student loan debt went past the $1 trillion mark on his watch.

Duncan is not without his supporters.

“Arne Duncan has distinguished himself as one of our best Secretaries of Education," said Senator Dick Durbin who, like Duncan, is an Illinois Democrat. "Always open to new ideas but never forsaking his commitment to our students, Arne brought the values he learned at home in Hyde Park and the experience of leading Chicago's public schools to serve our President in Washington."

Senate Education Committee chair Lamar Alexander (R-Tenn.) said Duncan was one of Obama's "best appointments.”

Education was expected to be a signature issue in the Obama presidency. But many of the initiatives that Obama championed and eventually blew up were rolled out during Duncan's tenure. The college ranking system, introduced by Obama in the summer of 2013, was panned by nearly everyone in higher ed and underwent a slow-motion collapse before it was shut down in September.

During the 2015 State of the Union Address, Obama inexplicably announced his intention remove the tax breaks from 529 College Savings Plans, only to reverse course days later when a firestorm erupted.

Critics who denounced the Bush administration's No Child Left Behind were chagrined when ED rolled out Race To The Top, where states competed for federal grants based the number of charter schools and teacher evaluations determined by student test scores. Race To The Top, which would leave many people at the bottom, paralleled the spirit, if not the letter, of No Child Left Behind and helped pave the way for the test-driven Common Core standards.

Duncan's imperiousness made him a lightning rod for criticism. In November 2013 he told a gathering of the Council of Chief State Schools Officers in Richmond, Va. that he found it "fascinating" that the most vociferous critics of the Common Core are "white suburban moms who--all of a sudden--their child isn't as brilliant as they thought they were and their school isn't quite as good as they thought and that's pretty scary."

Duncan later said he regretted his "clumsy phrasing" but offered no apology. The irony is that minorities and students from low-income backgrounds are widely believed to be at a greater disadvantage in standardized testing than their white suburban counterparts--a concept Duncan could dismiss out of hand. In a 2013 visit to Dunbar High School in Dayton, Ohio, he said, "The vast majority of kids don't drop out of high school because it's too hard. They drop out because it's too easy."

The Common Core standards have been implemented by the District of Columbia and 42 states, but it's not clear that the center will hold. Duncan's tenure saw the rise of an anti-Common Core, anti-testing backlash among liberals and conservatives alike; 20 states have adopted the Common Core with modifications, and 28 are adopting it incrementally, which could set the stage for eventual repeal. New York won't have full adoption until 2022. Indiana, Oklahoma, South Carolina and Tennessee have adopted anti-Common Core statutes, and Louisiana is expected to follow. Missouri, New Jersey, North Carolina and Utah are revaluating their Common Core compliance.

Some critics say that from a policy standpoint, Duncan was essentially a conservative disguised as a liberal, whose initiatives are part of a neo-liberal attempt to crush teachers' unions, privatize public education through charter schools and decree that standardized testing will be the last word on student and teacher evaluations. Studies by the Brookings Institute and the National Center for the Analysis of Longitudinal Data in Educational Research conclude that it’s not clear whether the Common Core has had any positive impact on student outcomes.

Duncan will be replaced on an interim basis by John B. King, Jr. A higher ed source who spoke on background said, "He's been delegated the duties of the Deputy Secretary of Education since January, so technically he was already the number two person in the Department."

Having King step in for Duncan allows Obama to avoid a confirmation fight in the Republican-controlled Senate--and have his education policies lambasted by Congress as he heads out the door. But it will not prevent new scrutiny of King's tenure a Secretary of Education in New York, where he has been slammed for implementing a state-wide testing regime that paved the way for the Common Core in the first large state to implement the program.

The reasons for Duncan's decision to leave now are not easy to discern. A higher ed source who spoke off the record said, "I wouldn't be surprised if he has another job already lined up. Political appointees tend to leave toward the end of an administration. Even if a Democrat wins the presidential election, a new administration will likely want its own cabinet members." The source pointed to the two-year period that Duncan will have to get through before he can lobby the ED, and his ED ties may be what future employers will be paying for.

An ED spokesperson could not be reached for comment.