Congress has increased the income guidelines for some of the students enrolled in the District's federally funded school voucher program, an effort aimed at preventing hundreds of students from being forced out of the program before it is evaluated.

The change allows the private school voucher program to increase the income ceiling from 200 to 300 percent of the federal poverty line for students who enrolled in 2004 and 2005, the first two years of the program, congressional aides said. The change was approved by the House of Representatives on Friday night and the Senate on Saturday morning.

The legislation covers 1,500 students who enrolled in the first two years and are involved in the study. Officials estimate that more than 300 students would have been forced out of the program under the original income guidelines.

Sally J. Sachar, president and chief executive of the Washington Scholarship Fund, which administers the voucher program, said the legislation will raise the income ceiling for a family of four from $40,000 to $60,000 a year. About 40 students left the program because household income exceeded the original guideline.

In 2004, Congress established the five-year experimental program to give low-income students in troubled D.C. public schools an opportunity to attend higher-performing private schools. With a $14 million annual federal allocation, the program covers tuition and other expenses for about 1,800 students, from kindergartners to high school seniors, attending 58 private schools.

Next spring, the U.S. Education Department will release the results of the first in a series of studies to determine whether students who receive the $7,500 vouchers are performing better than their public school peers. Sachar said students in the study group will be evaluated even if they have returned to public schools. A large exodus of students from the program, therefore, would defeat the purpose of the evaluation.

"Enabling the students to stay in the [program] is a priority if it can be done in a way that allows the program overall to serve low-income families, which this change does," Sachar said yesterday.

But Del. Eleanor Holmes Norton (D-D.C.) objected to the legislation, which was attached to the Tax Relief and Health Care Act of 2006.

In statement issued yesterday, Norton said the changes destroy "the low-income rationale for the program."