Although some middle and high schools in Washington, D.C., have installed metal detectors at their entrances, no other school system in the region has embraced the technology, even as metal detectors have multiplied in courthouses, museums and other public buildings across the region over the past two decades. Many school officials view metal detectors as costly, impractical and fallible. To suburban parents, they conjure up images of armed camps. Even at Albert Einstein High School in Kensington, Md., where three loaded guns were found in a locker last week, consensus is building against them.