Some Northern Virginia school districts, already struggling to hire enough teachers and administrators to keep pace with enrollment growth, are finding their problems compounded by large numbers of teachers and principals who are choosing to retire.

In Fairfax County, for example, officials are projecting that more than 400 teachers will retire this year, up from 325 last year. Officials in Arlington and Prince William counties also said they are seeing an increase in teacher retirements, although they did not have numbers available.

In addition, a dozen principals and more than 20 assistant principals in Fairfax have announced that they are leaving the system, most through retirement. At least nine principals will need to be replaced in Prince William and at least five in Arlington.

Administrators said the main reason for the trend is that school districts went on a hiring binge during the student enrollment boom of the late 1960s and early 1970s and many of the teachers and administrators hired during those years are now reaching retirement age.

The median age of teachers in public schools nationwide is 44 and has been rising steadily since the 1980s, according to the National Education Association.

Northern Virginia school officials said the increase in teacher retirements also appears to be attributable to a new state law that allows teachers with at least 30 years of service to retire at age 50 with full benefits instead of waiting until age 55. The law, which takes effect July 1, also applies to other state and local government employees in the Virginia Retirement System and was approved in response to lobbying by state employees.

Fairfax school officials have identified about 180 school employees who are eligible to take advantage of the early retirement option this year. Although that is a relatively small number of employees in a district the size of Fairfax, it exacerbates the hiring crunch, officials said.

"Exactly how many are retiring because of the change, we can't say yet. But it was already a tough market, and this certainly poses an additional challenge," said Kevin North, director of employment for Fairfax schools.

In contrast, Montgomery County school officials said the number of teacher retirements in their district appears to be holding steady this year.

School districts in the Washington area and nationwide are experiencing a critical teacher shortage. In addition to the problems caused by the aging work force, fewer people are entering the teaching profession. At the same time, the demand for teachers is growing as districts face record enrollment growth, institute new programs and try to reduce class sizes.

The shortages are particularly acute in areas such as special education and upper-level math and science courses. Fairfax, for example, on average, has only two applicants for every special education position, compared with 13 for every elementary teaching position.

Fairfax, which is the Washington area's largest school district and employs about 12,000 teachers, needs to hire about 1,600 teachers for the coming school year. Like many other districts, it has responded to the shortage by offering teaching contracts earlier. Fairfax has made about 600 offers, and 484 have been accepted. North anticipates the district will be able to fill all its slots before fall.

Montgomery has 641 new teachers under contract and needs to hire a total of about 1,200, said school system spokeswoman Kate Harrison.

Principal jobs are especially hard to fill because fewer teachers are expressing interest in the kinds of administrative jobs that lead to those positions, officials said.

"We are being hard pressed to fill all our administrative positions," said Prince William Superintendent Edward L. Kelly. "It's a difficult and demanding job, and the relatively small pay increase coupled with the headaches that come with it . . . well, we just have fewer people coming to us saying, `I want to be an administrator.' "

The Fairfax School Board will meet this month to discuss new ways to encourage teachers to become administrators, including expanding internship and mentoring programs and raising administrative salaries.