Calling in a bomb threat has always been a crime, not a prank. I see three possible conditions here:
1. He heard of a plot for a mass murder on the Virginia Tech campus and warned the community via Yik Yak
2. He was personally threatening to carry out a mass murder on the Virginia Tech campus and decided to go super villain style and announce it
3. Neither 1 or 2, He was pranking a mass murder threat on the Virginia Tech campus
If it's (1), then he's free and clear once he comes clean about all he knows of the plot for mass murder and there is credible evidence to show that such an event was immanent. If it's (2) then he's likely facing free room and board with roomates not of his choosing and very little outdoor time for the foreseeable future. If it's (3) it would fall under threats in the Virginia statutes, and is a class 6 Felony. Note that the law does not distinguish between an empty threat and a viable threat; when it is in writing (including digital communications) it meets the test of the law.