Unemployment in metropolitan Washington declined to 4.5 percent of the work force in September from 4.7 percent the previous month, with the return to school of students who worked or sought jobs in the summer.

Total area employment declined by 14,400 in September to 1.49 million as unemployment decreased 5,200 to 70,300.

The area's jobless rate of 4.5 percent, adjusted for normal seasonal variations in employment, was down from 5 percent one year earlier. Nationally, unemployment during September was 6 percent and that figure declined further to 5.8 percent in October.

In the District, which has the area's largest concentration of unemployed youths, the September jobless rate was 8.8 percent compared with 9.4 percent in August and 9.6 percent in September 1977.

About 29,000 D.C. residents were out of work in September compared with 32,000 the previous months as total employment declined to 300,500 from 305,400.

Over the past 12 months, areawide employment has increased 24,000 to the current 1.49 million level while in D.C. the number of jobs increased by 5,100, according to the D.C. Department of Labor.

Most employment gains in the past year have been in the retailing business (277,900 jobs compared with 268-800 last year), government (527,100 vs 514,700) and services (344,100 vs. 334,200).

Average weekly earnings of area manufacturing workers declined to $214.79 in September from $214.95 in August but were above the $213.72 level of the year ago.