The reason for shootings in schools is simple. They're sites where the attacker can be certain that there's little to nothing in the way of any way for people to resist. Someone with two machetes would go there for the same reason (and almost certainly do more damage. No reloading, and slash wounds are harder to treat than through holes. Most modern ammunition purchased is metal coated, so it goes straight through. Thus the constant repetition in safety courses of _always_ be aware of what's behind your target)

They're also large zones of stress and pressure - not just peer pressure, but intolerance from above as well. Thus the bomb threats, etc. I will say that we don't have as many street bombings as the UK has had in the past. (stuffed in the old metal rubbish bins) That may change after the Boston Marathon bombings. (interestingly enough, if the bins were shaped right, and strong enough at the base, they'd just channel the force of the blasts upwards. Showering everyone with old banana peels, bottles, cans, and half sandwiches. Downside? The trash cans would probably cost $3,000 each, and would require a truck with a small crane to move them around.)