Well, if you detect something may be wrong with a teen behavior, as a teacher you'd try to get to the bottom of it first, by talking to them and if you think the kid may be at risk, you'd signal them to child protective services so that it triggers an inquiry. So, the professionals there will eventually act.
Of course in some cases, they are too late or do too little (so many stories of abused children, even though "everyone knew" something was wrong), but that's no reason to turn a blind eye and hope someone else will take charge.