The Arlington School Board decided last night to hire 14 teachers -- probably from the ranks of those it previously fired -- because enrollment has surged ahead of predictions.

In Arlington, 322 more students than expected showed up for classes, chiefly because of an influx of foreign students. Elsewhere in Northern Virginia there were almost no enrollment surprises and enrollments continued to fall, as predicted.

The total number of students dropped in Arlington but not as much as expected. Instead of the predicted 5 percent decline, enrollment shrank only about 3 percent. That is the second lowest decrease in 10 years, the lowest having come in 1975 with a large influx of Vietnamese refugee children.

Arlington School Superintendent Charles E. Nunley said the figures "speak well for the Arlington schools." He attributed the slowed rate of decrease to several factors, including an increase in the county's foreign-born population and a slight increase in the birthrate, which he says the kindergarten and first grade enrollment figures are starting to reflect.

"I think there are also more people keeping their children in the public schools because of their changing attitude toward public schools," Nunley said.

The teachers to be rehired most likely will come from the ranks of 35 former Arlington teachers who were dismissed in a reduction in force last spring and not rehired.

Nunley said the funds for the additional teachers will come from a $313,000 contingency fund for increased enrollment set up by the County Board.

There are currently 14,710 students in 30 schools and one special education center in Arlington, compared with 15,146 last September.

Among factors cited as contributing to the enrollment decrease in Northern Virginia schools over the last decade are an increase in elderly and single-person households and the high number of condominium conversions, which have forced many young families to leave the area.

Most school systems there found this year's enrollments pretty much in line with what they had anticipated, although official student counts in most systems won't be taken until Sept. 30.

In Fairfax, the 11th largest school district in the country, the projected number of students for fall 1981 was 125,000, down 2,000 from last year. A count being taken this week was running close to initial projections. Prince William County's current enrollment of 35,422 represents a decline of 252 students from last fall and the Loudoun school population has decreased by 2 1/2 percent to 13,100.

In Alexandria, enrollment is down by about 5 percent, but the total was unavailable.