The question to be raised it how many of those students arrived for class after the first shooting, and were shot. You can certainly turn away traffic at the entrances, and use email / phone tree technology to inform people in short order that something is amiss.
In addition, why not just an old fashioned "air raid" siren type PA system, where vocal warnings could be broadcast in addition to siren type warnings.
Sad we've come to this, but it could have saved lives here.
The university I attended could not be “locked down” Several major thoroughfares run through the campus. There are also private residences and private businesses scattered through the campus proper.
A lock down on the dormitories could be done by the campus cops. But the area and the layout of the entire campus would be improbable in less than a few hours.
I do not know if the VT campus has a similar layout.