There is not sufficient evidence that metal detectors make schools safer.
Further, there could be significant costs to both install and maintain metal detectors, in addition to the security personnel that are needed to operate them (typically at least two per detector). Security experts have also expressed concern that the amount of time it takes to have students go through metal detectors is both impractical at the start of a school day and can create dangerous situations in which large groups of students are standing outside, potentially exposed to violence.
Most importantly, research has shown that metal detectors and other highly visible security measures actually increase student fear levels and decrease their sense of safety. When students do not feel safe in school, their social, academic, and personal wellbeing can be compromised.