Mass shootings on campus give rise to a new kind of life-saving service journalism: an anonymous message board called Sidechat

When a gunman began firing inside an academic building on the Brown University campus, students didn’t wait for official alerts warning of trouble. They got information almost instantly, in bits and bursts — through phones vibrating in pockets, messages from strangers, rumors that felt urgent because they might keep someone alive.

On Dec. 13 as the attack at the Ivy League institution played out during finals week, students took to Sidechat, an anonymous, campus-specific message board used widely at U.S. colleges...