Gov. Scott Walker said in a letter to U.S. Education Secretary Arne Duncan that he supports Wisconsin's application for a federal waiver from the No Child Left Behind law, but he has concerns with how it treats private vouchers schools.

Walker wrote in the letter, which was sent Wednesday, that he is concerned with what he calls a disparity in treatment between public schools and those in the Milwaukee parental choice program.

In the application, the Department of Public Instruction provided a ranking of Wisconsin's lowest-performing schools, including private voucher schools in Milwaukee. Inclusion of the voucher schools wasn't required by the federal government.

Tensions are high ever since Republican lawmakers didn't introduce a promised proposal to create a new accountability system that treats public and vouchers schools the same.

State education officials are hoping to join 11 other states that were already granted breathing room from the landmark educational requirements.

The No Child law sets strict testing rules and requires minimum proficiency levels, but the Obama administration is letting states seek waivers if they prove their standards are more rigorous.

State Superintendent Tony Evers said the state's waiver includes provisions for holding schools accountable in four areas: student achievement, student academic growth, narrowed achievement gaps and readiness for higher education.

Under the state's proposal, schools and districts would be given accountability ratings. Top schools would get public recognition and the worst schools could face closure.