PHOENIX -- Arizona schools are being shorted millions of dollars because of how federal officials parcel out funds to help students with limited English proficiency, according to a new report.

The General Accounting Office said the U.S. Department of Education relies on census data when dividing up the cash. But the report issued Thursday said that data appears to be inaccurate -- and sometimes by large margins.

In the case of Arizona, the GAO said the federal agency provided aid in the 2004-05 school year based on the state having only about 100,000 students classified as "English language learners.'' That produced slightly more than $16 million in funds from the federal No Child Left Behind Program for schools with those students.

But the GAO, the investigative arm of Congress, said figures from the state itself for that school year put the number at closer to 153,000.

"No Child Left Behind is a mandate,'' said U.S. Rep. Raul Grijalva, D-Ariz., one of the lawmakers who sought the GAO report, with the federal government requiring schools to ensure that students progress -- including by learning English. He said the study shows "it's become an unfunded mandate.''

State School Superintendent Tom Horne said the state has an accurate way of counting who is an English learner: They retain that status until they pass a test showing proficiency. And Horne said he believes the state's figures are accurate. "If the U.S. Department of Education is only funding us for 100,000 students, that is an injustice,'' he said.

Grijalva said the exact amount of additional dollars Arizona schools are owed cannot be directly computed. That's because thespecial funding is provided to qualified schools which apply for the grants.

For example, he said there is one school district in Pinal County where most children are on free or reduced-price lunch programs and most of the youngsters come from homes where English is not the primary language.

But the community that grew up around the school is a retirement community, meaning no kids, much less any who do not know English. Grijalva said the U.S. Census Bureau uses that data as a key indicator of eligibility for the federal dollars, with the result that the children in the school are not considered English learners.

"In reality you have a school with significant needs that now will not get funded because they don't meet the criteria,'' he said. Grijalva said other school districts are in a similar bind.

The GAO report says federal law allows the U.S. Department of Education to use state-provided data. But the study says the federal agency chooses not to, believing the census numbers are better.

Grijalva said the report proves that's not necessarily the case.

But it can work both ways.

Of the 12 states studied by the GAO, six, including Arizona, were shorted by the use of census data. But six others actually got more than state numbers suggest they were owed.

Horne said Congress needs to fix the problem.

Grijalva agreed. In fact, he said now that the Democrats will be taking control of both the House and Senate, the entire 6-year-old No Child Left Behind program will get real congressional oversight that he said it did not get with the Republicans in charge.

And Grijalva said the entire law may need to be rewritten to ensure that federal dollars go where there is a "real need.''