Threats against schools don't just come written on bathroom walls these days. They are spread using smartphone apps, social media and Internet phone services.

These anonymous reports of bombs or other threats of violence are forcing school evacuations and responses by police or other authorities. In the vast majority of cases, a threat turns out to be a hoax.

Still, the use of the modern technologies has made it that much harder to determine if threats are real and to find the culprits.

School safety experts say the number of such incidents appears to be increasing — as are the complexity of the cases. The latest figures from the National Center for Education Statistics, from the 2009-2010 school year, show 5,700 such disruptions.