The D.C. school voucher program has reached full capacity in its second year of operation, with 1,733 students attending private schools through the taxpayer-funded scholarships, officials announced yesterday.

But 47 students who won vouchers this spring were unable to use them because there were not enough high school spaces. And program administrators said the shortage threatens to get worse in future years, which would hurt efforts to evaluate the impact of vouchers on student achievement.

Under the $14 million-a-year program, low-income D.C. children receive federally funded grants of as much as $7,500 each to cover tuition and other expenses at private and religious schools in the city.

About 700 vouchers went unused in the 2004-2005 school year because of a lack of applicants, a situation that occurred because there was little time to publicize the scholarships after Congress approved the legislation, administrators said. In contrast, the program received 1.7 applications for each available voucher this year.

The U.S. Department of Education plans to evaluate the five-year pilot program by comparing the test scores of voucher recipients with the scores of public school students who sought the scholarships but lost out in a lottery. The number of applicants this year ensures that the latter group will be large enough for that study.

But officials with the Washington Scholarship Fund, the nonprofit organization that administers the program, said researchers will not be able to track the same students' scores over several years if the shortage of high school slots continues. Many voucher students now in elementary and middle school would be forced to leave the program, which would make the study results less meaningful, said Sally Sachar, the fund's president and chief executive.

"If too many students drop out of the program -- because of the high school capacity issue -- there is the risk that the evaluation results will be inconclusive," Sachar said.

The number of voucher slots being offered by private high schools is low because their tuition is often much higher than the $7,500 maximum amount of the federal grant.

Sachar said officials are working on several fronts to increase the number of slots. They are urging private elementary and middle schools that participate in the program to expand to the high school grades. She said they also are raising private money to fill the gap between the $7,500 voucher and the actual tuition rate. Such fundraising allowed five voucher students to enroll at private high schools this fall, Sachar said.

Sen. Sam Brownback (R-Kan.), chairman of the appropriations subcommittee on the District, proposed over the summer to address the shortage of slots by expanding the voucher program to private high schools in Maryland and Virginia. But he dropped the idea in July after critics, including Mayor Anthony A. Williams (D) and D.C. school board members, said the change could invalidate the results of the federal study.

To illustrate the crunch at the high school level, officials said that while 110 voucher students will enter ninth grade next fall, only about a half-dozen graduating 12th-graders will have left the program. And in fall 2007, the number of voucher students entering ninth grade will rise to 162.

One private school in the program, Rock Creek International in Northwest Washington, has agreed to expand to 9th and 10th grades next year.

Danny Hollinger, head of the school, said Rock Creek officials had been thinking about such an expansion for several years. The school will add 11th- and 12th-grade classes in subsequent years, he said.

Of the 1,733 students now receiving vouchers, 891 were enrolled last year; 38 won scholarships last year but decided to wait a year to use them; and 804 won their grants this spring.

Sachar said that 90 voucher students from the first year decided not to re-enroll.