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A public high school can go on lockdown because the schedule is the same, pretty much, for everyone. Nearly everyone is in their classrooms at the same time. (Speaking of which, when you say go on lockdown, do you mean make people stay in their classrooms? Because that obviously wouldn't have helped in this case.)
In a college setting, some people are in the library while others are sitting on the quad while others are eating while others are in class while others are in bed sleeping while others are arriving and still others don't even have class that day. For this reason, comparing VT to a small city makes more sense than comparing it to a high school.