Idk if we’d be so angry if the system stopped an angry student from committing an actual shooting. Sounds like the system would have correctly notified authorities and alerted to an actual threat of one occured.
A teacher should know the sensitivity of such a statement, joke or not. Some things you don’t joke about.
At the end of the day, tools like these are helpful when used in their intended way, but are rife with potential for misuse.