It sounded simple enough: Help low-income students perform better in public schools deemed in need of improvement by giving them tutors. And let the federal government pick up the tab.

But what appeared to be an easy way to address a component of President Bush's No Child Left Behind Act has become anything but. The federal government, state education departments, local school systems and many of the 1,700 or so private education companies offering tutoring are battling over complex rules. Just who can tutor what, to whom -- and where?

"Nobody really knows how effective the providers are in a highly valid way with regard to helping kids," said Steven M. Ross, director of the Center for Research in Educational Policy at the University of Memphis. He is helping several states devise statewide assessment tools.

Under No Child Left Behind, schools that fail to make adequate yearly progress and are deemed in need of improvement must offer eligible parents a choice: Change schools or use free after-school tutoring services. Eligibility is offered to low-income children who attend Title I schools, institutions at which a percentage of children qualify for free and reduced-price lunches on the basis of family income.

The number of students receiving tutoring under supplemental services is expected to exceed last year's estimated 300,000, and accountability has emerged as the key issue.

Tutoring companies have said they can prove that they are effective. And some parents have said their children are being helped. Harriet Davis of the District said her son, Kasey, 11, got slightly better grades at year's end at Stuart-Hobson Middle School because of two hours a week of after-school tutoring by Catapult Learning, a leading private Supplemental Education Services provider. Kasey said he thought his reading comprehension improved.

State officials have said they need broader, more precise measurements. The first systemwide effort to attempt an evaluation was released recently by the Chicago school system, which had more than 60,000 students in 343 schools tutored under Supplemental Education Services in 2004-05. Officials concluded that they could not make broad conclusions about whether the tutoring programs were working.

"This is the issue," said Jeff Cohen, president of Catapult Learning. "If we get over this issue and get consensus about whether or not we can call a program successful, we can get to work."

But getting to work is proving difficult. Smaller issues, school administrators have said, are causing major headaches.

Should private tutors, for example, have access to school buildings? They do in the District because children can be tutored after school, Supplemental Education Services coordinator Tamika Maultsby said. In Los Angeles, however, tutors will not have such access next year; SES coordinator Becki Robinson said there is no room because the L.A. Unified School District runs so many other after-school programs.

Another issue: Can a school system break its contract with a private company whose tutors do not show up? Chicago school officials this year learned the hard way that the answer is complicated. Ginger Reynolds, the Illinois assistant superintendent for teaching and learning, said it is up to the state -- not the local school system -- to approve providers and remove them from the list. So Chicago could kick the providers out of only a few schools for flagrant violations. As a result, the state just issued guidelines to strengthen oversight of providers.

"It's incredibly difficult to implement this program," said Elizabeth Swanson, director of after-school programs in Chicago public schools. "This year, we are going to have 230,000 eligible children in 400 schools."

Supplemental Education Services supporters, including Bush administration officials and the private companies that compete to offer tutoring services, said they are confident that the issues can be worked out and that children will benefit.

"It's an innovation, and with most innovations, it takes awhile for state and local officials to understand how to best put in place parameters that encourage innovative practices while at the same time holding providers accountable for results," said Nina S. Rees, assistant deputy secretary for the U.S. Education Department's office of innovation and improvement.

(Kaplan Inc., a subsidiary of The Washington Post Co., provides tutoring under SES.)

Critics have said that although the idea behind Supplemental Education Services is laudable, the Bush administration's rules are tilted in favor of the private sector.

"The Bush administration very much wants to encourage private [companies] to get involved in elementary and secondary public education," said Jack Jenning, director of the nonprofit Center on Education Policy, a Washington-based advocacy and research group. "They are finding any way they can to encourage federal money to be shifted to private companies . . . . Their whole bias is in favor of private companies and against school districts."

The Department of Education issued regulations, as well as non-regulatory guidance, in June to help states and school systems smooth over the difficulties as they implement the program. Contentiousness remains, however.

Supplemental Education Services administrators said that some of the private and nonprofit companies approved by the states are doing a great job of complying with the rules and helping students improve in reading and math.

They complain, though, about some for-profit companies, including those that agreed to work in a school and canceled when too few students signed up, leaving parents in the lurch. Maultsby said that in some instances, D.C. parents -- asked to make three choices on their application -- had all three providers opt out. Some school systems are changing rules to prevent that.

Another issue, providers said, is some companies expect schools to provide books for tutoring, although the law says the companies should have their own programs. And competition among providers has become so intense that providers were found wandering school grounds to recruit students.

Private providers and Education Department officials, however, said that many school systems simply do not like the program and throw up roadblocks to implementation.