The line of young, eager teen-agers snaked out the door of the D.C. Employment Services Building yesterday and down the block, and at noon there was still an hour's wait to sign up on the last day of registration for the District's summer jobs program.

"As long as I get a job, it will be worth the wait," said Monique Lindsay, a 15-year-old sophomore at Roosevelt High School, as she stood patiently at the end of the line.

Unlike in past years, when not everyone who signed up got a job, this year Monique and other teen-agers in the line did not have to worry about being left out.

Mayor Marion Barry announced 10 days ago that he would guarantee summer jobs for all 14- through 18-year-olds who signed up for the program by yesterday's deadline. He later expanded the guarantee to include 19- through 21-year-olds.

But despite the guarantee, announced and promoted by city officials with much fanfare, about 2,000 fewer teen-agers signed up for jobs this year than last year's 23,800.

When the mayor announced his guarantee, he said city officials had established a registration goal of 25,000. By yesterday's deadline, the city had registered a total of 21,661, according to Matthew Shannon, director of the D.C. Department of Employment Services. He added that when registrations through community groups trickle in, the figure should go to 22,000.

"We wondered why, too," said Shannon, when asked why the registration fell short of last year's despite the promotion efforts.

The greatest factor was an expanded summer school program that is competing with the jobs program this year, he said. "That has impacted us very, very heavily."

Shannon said city officials went back to some District schools three and four times to try to register students.

For example, they returned to Dunbar High School, where the mayor had announced the job guarantee, and tried to find the students in the audience who had given their names that day to sign them up, he said. They announced the names over the loudspeaker and stayed all day, but only two of the students showed up, Shannon said.

City officials now plan to continue some of their outreach efforts beyond yesterday's official registration deadline for young people in families receiving Aid to Families with Dependent Children and in public housing, Shannon said.

The program runs from July 1 to Aug. 16 and provides jobs paying at least the federal minimum wage of $3.35 an hour for at least 20 hours a week. Most of the jobs are in government agencies or with nonprofit organizations, and the city pays the entire salary.

The summer jobs program this year is expected to cost the District $6.8 million, and the federal government is expected to provide another $5.7 million for it.

Private employers also can participate in the program, at their own expense, by signing up to hire youths sent to them by the city.

The young people signing up yesterday included some with well-formed career goals and some with only vague ideas on what they want in their futures.

Monique Lindsay said she hoped to get a job as a lab technician because she wants to enter the medical field, particularly pediatrics.

Patrice Gillis, 14, who attends Francis Junior High School, said she had no particular field in mind but that she wants to get a clerk-typist job somewhere this summer.

And at least one in yesterday's line has political ambitions.

Oliver Roy, 18, was signing up for his third year in the city's summer jobs program, which he says has been "great" for him. A student at McKinley High School and president there of Students Against Drunk Driving, Roy said he would ask if there are any politically oriented jobs available this year.

He said he wants to go to Howard University to study prelaw and political science and then run for city office, perhaps for a seat on the D.C. City Council.

With a smile, Roy gave a succinct explanation of why his career ambitions have turned to politics: "It's fun, it pays good money, and the people are weird."