Why Not Metal Detectors?

After the discussion in Helena about school safety, I don't understand why metal detectors were never in the conversation. You must prevent the weapon from getting in the school, PERIOD. I checked: top-of-the-line walk through detectors range from $2,100 to $4,200. We have money for new backboards or football lights or turf, but not detectors? They are in use everywhere now to detect weapons; why not where kids are conducting the carnage? Wouldn't be great if our school detectors never found a weapon? That makes it worth it.