Springfield Estates Elementary in Fairfax County was struck by arsonists over the weekend, the third time in five years the school has been set a fire.

The fire, confined to the physical education room and a hallway, caused $15,000 to $20,000 damage before it was brought under control early yesterday, according to the county fire marshal's office. All classes, except physical education, went on as scheduled.

A fire marshal's spokesman said, "we have suspects" who are also believed to be responsible for a fire at the school in January which caused $30,000 damage. No one has been charged.

In 1972, a juvenile was convicted of setting a blaze at the school that caused $125,000 damage. The youth served about a year in a juvenile facility, but he is not a suspect the fires set this year, the fire marshal's spokesman said.

Firefighters responded to yesterday's blaze at 2:37 a.m. after a call from school security officials, who had gone to the school earlier to investigate an alarm, apparently sounded when arsonists broke into the school.

In related developments, four schools in the southeastern part of Fairfax County were hit Saturday night by vandals who caused $1,612 damage in broken windows and an estimated $5,700 damage to school buses.

Windows were broken at Edison and Lee highs, Francis Scott Key In termediate and Forestdale Elementary. Bus windows at Lee were broken, tires slashed and radiators punctured.

The vandalism triggered a sound alarm at Edison, but such an alarm - which can be activated by noise of pipes creaking in a temperature change - does not have a high priority in response by school security forces. As a result, by the time security officers arrived, Fairfax police, on routine patrol, were already there.

The school system's security force of 14 has to deal with "several hundred" sound alarms many nights of the year, and most of the alarms do not involve intruders, according to Sgt. Edward Cox, head of the force.