Principal James Howell snapped his fingers and the 200 students sitting in Taft Junior High School's auditorium snapped to attention -- all except one seventh grader who continued talking.

"You're going to be back home before the first 10 minutes of this brand new school year if you don't stop that noise, young lady," Howell warned. The chatter ceased.

And with that, school was in session.

From one end of town to the other, the District's approximately 88,700 students, from age 4 to late teens, and 5,700 teachers -- some neophytes and others expecting to retire next June -- put summer behind them and faced the new semester.

As youngsters clad in the stiff newness of cordouroy and denim and toting bright nylon backpacks streamed in from the cool pre-autumn morning, teachers and school administrators pondered the challenges of instituting several new programs designed to reduce the city's truancy rate, increase achievement on the secondary level and improve teacher morale and performance. Those were some of the concerns outlined by D.C. schools Superintendent Floretta Dukes McKenzie during an interview earlier this week.

McKenzie leaned back in a chair in her penthouse office downtown, smiled broadly and crossed her fingers.

"The school system is like an airplane, she said. "The hardest part is taking off and landing.

"We're busy making sure we have our staff in place, all the materials are where they should be, and the buses for special students are ready to go, and meals are ready to be prepared and delivered," McKenzie said. "School people are usually quite excited about the new year. It's an opportunity to do something better than last year."

For some teachers and school employes, bright optimism about the new year has been dimmed somewhat by old problems including student discipline, decaying buildings, asbestos hazards, shortages of supplies and a $338 million budget request that is awaiting approval on Capitol Hill.

All school and administrative buildings in the school system reopened yesteday, though areas of asbestos contamination in 121 buildings have been sealed off. Contamination from crumbling asbestos, a cancer-causing substance, has been corrected in 41 other schools and administrative buildings, officials said.

The proposed 1985 budget, for the fiscal year that begins Oct. 1, includes a $1.5 million allocation set aside to reduce the school's rampant truancy problem, a $1.9 million line item to expand the system's pre-kindergarten education program and $350,000 for an incentive program to increase teacher productivity. The budget, as submitted to Congress, increased by $12 million over the $326 million passed by Congress last year.

In a recent meeting with school administrators and teachers at Woodson High School, McKenzie stressed among goals for this year reducing the truancy rate, which was 11.8 percent last year.

"If we have programs in our schools that are speaking to the needs of our students, that are demanding enough and delivered by people who care, we wouldn't have the numbers of truants that we have now. We wouldn't have massive class-cutting in some high schools," McKenzie said.

D.C. elementary schools have seen sharp improvements in student achievement in recent years. The rise in test scores has been attributed to the institution in 1980 of a Student Progress Plan that involved the establishment of required promotional standards in reading and mathematics.

This year, the plan will be instituted in seventh grade classes as well as in elementary schools, school officials said.

While the teachers were busy launching their courses yesterday, principals and their staffs tried to iron out scheduling and other snafus.

One lanky youngster lounged in the Taft lobby yesterday morning waiting to find out what high school he would attend. Meanwhile, a pregnant 14-year-old reported to Taft.

The girl, a ninth-grader, said she was due in a month.

"I'm ready to come back to school but I can't come back yet because I'm getting ready to have the baby," she said. "I'm in school today to find out whose class I'm going to be in when I come back."

"We've got a lot of pregnant girls in our schools," said Taft acting assistant principal Brenda Richards, who noted it is a growing crisis in many urban, black cities.

Despite the problems of individual students and the massive system that serves them, yesterday, the first of a 182-day school calendar, was a day of bright hopes and high ambitions.

In a speech that typified hundreds of first-day orations, John Nichols, 61, stood before a handful of students in his Taft Junior High typing class and declared, "We're going to have great year!"