Lawmakers on both sides of the aisle agree that the No Child Left Behind Act needs to be fixed, but that's where the consensus ends. The Senate starts debating its revamped version of the education law for the first time in 14 years this week, which will need to also pass in the House before it becomes law.

No Child Left Behind, the version of the Elementary and Secondary Education Act passed under President George W. Bush in 2001, has been criticized for giving the federal government too much authority over state education systems and for too harshly judging minority students. It required schools to make yearly progress -- and prove it with annual test scores -- with the goal of 100 percent student proficiency by 2014. When states couldn't cope, the government issued waivers exempting them from the most stringent requirements. Meanwhile, the deadline has come and gone without results, frustrating conservatives and progressives alike.

Leaders from both parties have a chance now to repair the law, but it's not easy. They're faced with figuring out how to reel in the government while keeping states in line, measure student performance while preventing overtesting, address gaps in achievement while catering to disadvantaged kids, and more.

Their solutions to these dilemmas have varied, and the discussion has finally reached a head. The Senate began considering the Every Child Achieves Act, which was passed by the Senate Health, Education, Labor and Pensions Committee in April, Tuesday afternoon. The bipartisan deal brokered by Sen. Lamar Alexander, R-Tenn., and Sen. Patty Murray, D-Wash., would give more power to state education systems.

The House version, called the Student Success Act, would do the same. The House was expected to take up the measure Wednesday, five months after it was forced to table the legislation because leaders encountered trouble drumming up support. Some conservative representatives expressed concern that it didn't do enough to limit the role of the federal government in schools.

Overall, Congress is trying to divide power -- and responsibility -- between the federal government and the states. Specifically, they're disagreeing over how often students get tested, what those results are used for, what happens when those results aren't high enough and how funding for low-income students works.

Here are three of the most buzzed-about topics expected to come up during the debates and why they matter:

Accountability: For years, white students have performed significantly better than their non-white peers, excluding Asians, by nearly every measurement in every state. In 2012-2013, the most recent academic year for which the Education Department has released data, 86.6 percent of white students graduated from high school. Only 73.3 percent of low-income students did. About 71 percent of black students graduated, and the numbers dropped to roughly 61 percent for disabled and English-learning students.

With the No Child Left Behind revamp, holding schools responsible for their students' success has been one of the biggest sticking points for Democrats. The Obama administration wants to see states have a plan in place for schools with large groups of failing low-income, minority, disabled and English-learning students, but Republicans are wary of further increasing the federal government's role in education.

Both the House and Senate bills reference accountability in vague terms. The two bills don't spell out what the interventions for low-performing schools would be, just that districts and states would have to address their weaknesses. The interventions would likely take the form of more dollars or support programs.

President Barack Obama isn't happy with either the Senate or House plans because of the lack of details. On Monday, Domestic Policy Council Director Cecilia Muñoz told reporters the president couldn't support either bill because they didn't have appropriate accountability standards.

RTR2XK5R U.S. President Barack Obama talks about providing states flexibility under reauthorization of the No Child Left Behind Act as Secretary of Education Arne Duncan applauds in the East Room of the White House on Feb. 9, 2012. Photo: Reuters

Portability: School choice is a big issue for Republicans, who say families should be able to pick where their children learn. But critics of portability -- a policy that lets federal funds follow poor students wherever they enroll -- argue that such measures to allow more choice are really just a ploy to undermine public schools, especially in low-income, minority neighborhoods. The debate is not about vouchers or charters schools, just more flexibility when it comes to school choice. 

Schools currently get Title I financial assistance based on how many low-income students they enroll and how concentrated they are in an area. Republicans want to change this to allow for portability. Democrats counter that this could lead to schools in bad neighborhoods being underfunded and wealthy schools getting money they don't need.

William Mathis, the managing director of the nonprofit National Education Policy Center at the University of Colorado in Boulder, said Democrats also think portability is too similar to the voucher system, which lets families use public funding to pay for private schools. Progressives have historically opposed it on grounds it hurts public schools.

With portability, schools with large low-income populations could lose an average of $85 per student, according to a February analysis from the Center for American Progress. Poorer districts, like New York City, could lose up to $116 million total, while more affluent ones, like Montgomery County, Maryland, could gain up to $3.4 million.

The House bill includes portability, but the Senate bill does not. The Washington Post predicted that Sen. Tim Scott, R-S.C., was likely to propose an amendment to include it in the latter.

Testing: Standardized tests have increasingly become an emotional issue for families across the nation, with parents and educators insisting that such measures often hurt low-income and minority children, as well as students who simply don't think like everyone else. Lawmakers are listening, especially since test results are often tied to school dollars.

Both versions of the No Child Left Behind overhaul specifically forbid the Education Department from making rules about state standards. That means the final reauthorization likely won't touch Common Core, the controversial national set of education standards that has been criticized for its rocky rollout and one-size-fits-all requirements. But the exams linked to the guidelines could change.

Both the House and Senate bills keep annual testing. Education Secretary Arne Duncan previously said that "parents, teachers and students have both the right and the absolute need to know how much progress all students are making each year towards college- and career-readiness," but groups including the National Education Association, the nation's largest teachers union, have pushed back.

Sen. John Tester, D-Mont., has said he'll introduce an amendment to try to reduce testing to once for math and reading in each stage of schooling -- elementary, middle and high school. There could also be some discussion around whether parents have the right to opt their kids out of the exams, a practice that's become a popular way for parents to protest how much test prep students undergo. An amendment from Rep. Matt Salmon, R-Ariz., would explicitly legalize opting out and make it so schools aren't punished for low participation rates.

Wealthy and low-income families alike have found themselves fighting the high-stakes exams, which they say lead to situations like the 2009 Atlanta Public Schools cheating scandal. More than 150 teachers and principals were found to have changed students' answers in an effort to boost their scores and meet academic progress targets. Eleven people were convicted on racketeering charges stemming from the scandal in April.

With these issues and others, it's difficult to predict how long it could take for both chambers of Congress to debate and pass their versions of the legislation, which would then go to conference. If the House OKs its bill by the end of the week and the Senate does so by the end of the month, the final reauthorization could be on Obama's desk by the end of the year.