The jury is still out on the effectiveness of metal detectors from what I've seen. They can be effective if all other ingress and even egress are addressed, as well. In one high school I visited a couple years back, there was armed security at the front door. My initial thought was that this could work. After my meeting in the school, one of the hosts showed me around, and was totally embarrassed when we came across a door at the back of the school that was ajar. School policy stated that all doors except for the front entrance had to be closed and inaccessible from the outside during school hours. Policies are only as good as those folks responsible for their implementation. I personally like the idea of metal detectors in some schools, especially in high crime environments. I'm not so sure they'd go over very well in more rural settings, though.
