A key Republican senator withdrew his proposal yesterday to allow District voucher students to attend private high schools in Maryland and Virginia, saying there was not enough time to pass legislation to help about 50 students who lack spaces for this fall.

"I will not be pursuing any changes to the District of Columbia school voucher program at this time," Sen. Sam Brownback (R-Kan.), chairman of the Appropriations subcommittee on the District, said in a statement. "With next fall's school season quickly approaching, it's become clear it would be difficult to implement any changes that would have a positive impact before students returned to class."

He added that he remains concerned about the shortage of high school slots in the program and plans to hold a hearing to discuss other ways to address the problem, which he noted is expected to worsen in the next few years.

The year-old federal voucher program provides taxpayer-funded grants of up to $7,500 for low-income District children to attend private or religious schools in the city.

In addition to letting students use their vouchers at high schools outside the city, Brownback had proposed increasing the annual $7,500 limit.

But the proposal drew criticism from Mayor Anthony A. Williams (D), Board of Education President Peggy Cooper Cafritz and Democrats on the Appropriations subcommittee. Critics said that making changes in the middle of the five-year pilot program could invalidate the results of a U.S. Department of Education evaluation of the voucher experiment.

"I'm pleased that Senator Brownback agrees with me that changing our city's school choice program at this early stage of its operation would be inadvisable," Williams said in a statement. "I'm glad that we agree that small changes now may indeed lead to unintended consequences later."

Sally J. Sachar, president and chief executive of the Washington Scholarship Fund, the nonprofit group that administers the program, said the shortage of high school slots can be attributed to the cost of high school tuition, which often surpasses the maximum $7,500 grant.

Voucher students can obtain scholarships from the schools to make up the difference, she said. But by the time the program has selected the voucher students, she said, many schools have already awarded their scholarship money.

Voucher recipients are selected through a lottery, based on the available spaces at participating private schools. But Sachar said the number of slots can change later, as schools make adjustments in how many students they can accommodate and as some schools prove more popular than others.

At this point, about 50 students who won grants by lottery this spring are unable to use them, she said. Program officials are still trying to place them, but many will probably have to wait until fall 2006 to enroll, she said.

Sachar said that starting next year, the program will stop accepting new high school applicants. Even with that change, she said, there will not be enough slots to accommodate the voucher students currently in elementary school once they move on to the upper grades.

"We need to get together with the mayor, congressional leaders, federal officials and business and education leaders in the District and explore what's available to us in terms of options," Sachar said.

"I hope a solution can come from that process."