Administrators at Oak Hill High School in Fayette County, West Virginia, are attuned to potential violence.

If a student scrawls a threat on the bathroom wall about shooting someone, which happens in schools on occasion, staff will set up a mobile unit of metal detectors in the school’s yellow-brick entrance way. Since April, though, the metal detectors have been replaced by slimmer-looking scanners that use ultralow frequency magnetic fields to scan students’ bags and pockets for weapons.

The detectors, sold by a publicly traded security company in Waltham, Massachusetts, called Evolv Technology Holdings, use algorithms that have been trained to identify any kind of gun or knife. If the machines do spot something, they will draw a box around an image of the suspected student and alert school officials. The system costs about $30,000 a year to use, according to Gary Hough, superintendent of the Fayette County school district.