About 20,000 D.C. schoolchildren have enrolled in summer school programs--17 percent fewer than last year and less than two-thirds the number the school system had prepared for.

School officials had expected as many as 35,000 youngsters--nearly half the city's public school students. Instead, about 30 percent of the city's school population has shown up for the morning sessions that began Monday in 132 sites across the city.

D.C. Superintendent Arlene Ackerman said class size in the remedial and enrichment programs will shrink from a planned 15 students to about 12 because of the lower-than-anticipated turnout. She called the situation "a great opportunity" for students to have more one-on-one instruction.

"What I wanted, more than anything else, was to be prepared," Ackerman said, adding that a small increase in standardized test scores meant that slightly fewer students were required to attend the summer session. "If we have 20,000, I'm happy with that."

School officials are still collecting data from individual schools and said they will not have a final enrollment figure until today or next week. Although enrollment officially closed Wednesday, several educators said they will not turn away interested students as long as they have space.

Last year, the District joined Chicago and other cities in launching massive summer programs for struggling students. School officials planned for 20,000 students last summer and were overwhelmed when more than 24,000 showed up. This year, other Washington area school districts are enlarging their summer programs to boost scores on increasingly important standardized tests.

Ackerman has touted summer school--along with Saturday classes, teacher training and other initiatives--as central to her effort to reform one of the nation's most dysfunctional school systems.

Yesterday, she toured summer classes at Brent Elementary School on Capitol Hill with U.S. Education Secretary Richard W. Riley, New York City School Chancellor Rudolph Crew and others before a round-table discussion on how to end the promotion of students not prepared for the next grade.

D.C. students are required to attend summer school if they score at the lowest level in math and reading on the Stanford 9 Achievement Test, and they are strongly encouraged to attend if they score poorly in one of the two areas.

A new program this year was designed for 5,000 top-achieving students, but it was not finalized until the very end of the school year and failed to attract even half that number. Ackerman said that the announcement of the program was delayed because the city's procurement office did not process a request until the last minute. She said that the program will be offered again next year and that parents will be notified much earlier.

Neither D.C. Council member Kevin P. Chavous (D-Ward 7), chairman of the education committee, nor school board member Tonya Vidal Kinlow (At Large) expressed concern that summer school enrollment dropped from last year.

"Twenty thousand is a lot," Kinlow said. "There are a lot of resources out here for children, and so perhaps parents are using alternative options."

Parents, students and educators have expressed confusion about who is required to attend summer school and whether Stanford 9 scores affect whether a student is promoted.

Officially, the test scores are a factor in promotion only for elementary school students. But Cardozo High School student Kathy Delcid received a letter Saturday saying she had to attend summer school because her Stanford 9 scores were too low for her to move on to 11th grade.

"It was just a surprise," said Delcid, 16, who has not yet received her report card. "I thought our Stanford 9 scores didn't have anything to do with passing the grade."