You don't see any difference between alerting law enforcement when someone threatens to shoot students, and the Nazis forcing people to rat on each other?
I bet you'd be pissed at Snapchat for not alerting the cops if a school shooting actually happened and they sat on the information.
This is the US. School shootings are a huge problem, and threats of doing it should be taken seriously until it can be determined no threat exists. The teacher fucked around and found that out.