When Washington's 89,900 public school students troop back to classes Sept. 6, many will find that, more than ever, high technology and private grants are influencing their educations.

More computers will be used this year for everything from career training and remedial help for students to record keeping, school officials said. Private corporations and associations will pay part of the costs for some of those programs and bring in teachers, curriculum and equipment for others.

A $500,000 grant from the Xerox Corp., for example, will be used to open a computer information and word processing career program at Coolidge High School. The program is designed to help students prepare for entry-level jobs in the computer field, according to school officials.

"When you start focusing on the act of teaching and learning, technology frees teachers to focus on instruction," said associate superintendent for instruction James Guines. "In my mind it gives you more time to focus and cause significant improvements in learning."

Based on the teachers union contract, the school day will be 30 minutes longer this year, running from 8:30 a.m. to 3:30. Last year's schedule was 8:45 a.m. to 3:15.

The expected enrollment of 89,900 is a decrease from 91,828 last year. It represents the smallest enrollment decline in 13 years, said school officials.

The D.C. school board last spring approved the area's first computer literacy requirement for students and teachers, but the resolution, which calls for developing a working knowledge of computers and their application and use, won't affect students until the 1987-88 school year. Teachers must be "computer literate" within their five-year recertification process.

Several other programs involving computers will take effect this year, however. A computer laboratory for remedial students who are far behind in their work was piloted at Spingarn High School last year and four more will open this fall at Woodson, Roosevelt, McKinley and Eastern high schools, Guines said.

"In terms of gains, we found that it is possible to raise a student's reading level five to six years in one year," Guines said, referring to students who are years below grade level.

The American Academy for the Advancement of Sciences will divide $1.6 million in grants among the District and three other school jurisdictions this fall. The money will provide curriculum and teacher training to improve science instruction in each of the city's junior high schools, said school system spokeswoman Janis Cromer.

In addition, Beers Elementary in Southeast has been selected as the site of a "model for elementary science instruction" by the National Science Teachers Association, according to school officials. The NSTA is expected to provide equipment and will bring in so-called "master teachers" on sabbatical from other parts of the country to teach at Beers, the officials said.

Some teachers, too, will bring changes to their classrooms. Twenty-one instructors on summer "externships" observed employes at work places including D.C. General Hospital, IBM and Pepco in order to share their experiences this fall with students who might have career interests in the fields represented by those employers. Another 33 are revising existing courses or designing new ones, in conjunction with employers, to make them more relevant to the work place for students.

An automated records system that is expected to eliminate a lot of paperwork and make student record retrieval easier will be piloted at eight elementary schools this fall and implemented at each of the city's elementary schools in the spring semester, said Cromer.

A project with the Virginia-based National University of the Air will start this fall as well. Using a satellite receiver at McKinley High School, instructors will be able to view teacher training techniques on videotape via closed circuit television, officials said.

A Senate proposal that the schools receive $1.5 million to study merit pay for city teachers is on hold with the rest of the District's fiscal 1984 budget until after the congressional recess. The Senate Appropriations Committee voted to add that amount as part of an additional $25 million funding package for the city.

For the past three years, some students at Lincoln Junior High have participated in a pilot version of a Youth Awareness Program, in which D.C. police officers help counsel youths on making intelligent choices about drugs, alcohol and sex. This fall, the program will be expanded to include 250 to 300 youths at Kramer Junior High and Wilson and Ballou high schools, school officials said.

About 90 teen-agers from around the city, 40 of them pregnant students, participated in a summer session of the program at Dunbar High.