Unemployment in Virginia rose to 4.3 percent in September, largely because students recently settled in school began seeking after-school work, the Virginia Employment Commission reported yesterday.

The rate was nearly 0.5 percentage point higher than it was in September 1989, when the state's economy was growing faster than it is now.

It rose 0.2 percentage point from August to September 1990, but the increase appears to have been almost entirely due to seasonal factors. The figures are not adjusted for seasonal variations, and student job-seekers are counted as unemployed even if they are looking for part-time employment.

There was one dark note in the numbers: For the first time in recent memory, the number of jobs in Northern Virginia fell over a 12-month period. From September 1989 to September 1990, jobs in the region declined by 1,700, to 778,300.

William Mezger, a research economist at the employment commission, said he did not believe Northern Virginia lost jobs even during the depths of the 1981-1982 recession. Much of the loss, he said, was in the construction industry. Employment figures for the entire metro area, to be released Friday, may also show a job loss.

Unemployment in Northern Virginia was 2.4 percent, the lowest of any region in the state but 0.5 percentage point higher than it was a year ago.

A total of 139,100 Virginia workers were unemployed last month, an increase of 7,600 from August and 17,100 more than September 1989.