WASHINGTON >> President Barack Obama asked Congress today to rewrite the nation’s main education law before the new school year starts in September, setting an ambitious timetable for lawmakers whose primary focus now is on budget cuts and the deficit.

At the same time, Obama issued his most detailed outline yet for changes to the No Child Left Behind law.

Obama said the law, enacted in 2002 under George W. Bush, got some things right but that it also got some things wrong.

“The goals of NCLB were the right goals,” Obama said, mentioning the law’s promises of putting quality teachers in every classroom, putting in place higher standards for learning, requiring accountability and highlighting achievement gaps among students.

“That’s the right thing to do,” he said at an Arlington, Va., middle school. “But what hasn’t worked is denying teachers, schools and states what they need to meet these goals.” He said that’s why the law needs to be rewritten.

“In the 21st century, it’s not enough to leave no child behind. We need to help every child get ahead,” Obama said.

The president has met several times in recent weeks with a bipartisan group of House and Senate lawmakers who would be key players in any rewrite. In Monday’s remarks, he set the start of the new school year as a deadline for Congress to send him a bill he can sign into law.

“I want every child in this country to head back to school in the fall knowing that their education is America’s priority,” Obama said.

Both Republicans and Democrats agree that the law needs to be rewritten; they disagree on the federal government’s role in education as well as on how best to turn around failing schools.

Education Secretary Arne Duncan said last week that the percentage of schools labeled as “failing” under the law and not meeting yearly targets for student proficiency in math and reading could skyrocket dramatically this year, jumping from 37 percent to 82 percent as states raise standards to try to satisfy the law’s mandates, according to Department of Education estimates.

The law requires states to aim to have all students proficient in math and science by 2014, a standard now viewed as unrealistic.

In his remarks at Kenmore Middle School, Obama said he wants an updated education law to empower principals and teachers, support innovation at the state and local levels, and target resources to schools with consistent records of poor performance.

Instead of labeling more and more schools as “failing” under the law, he wants a more flexible system that focuses on preparing graduating students for college and career and he wants better assessments to understand whether kids are meeting that goal. Proficiency in math and science will continue to be emphasized, Obama said, but he added that skills such as critical thinking and collaboration are also important.

Obama also said he wants to see a better effort at preparing and supporting teachers, holding them accountable for student progress and not making excuses for the occasional bad teachers.

“These are the steps we’re going to have to take to fix” No Child Left Behind, he said.