The trench coat by itself? Probably not a red flag. The trench coat combined with a seeming obsession with Columbine (if this is true)? Might be worth looking into.Come on... this stuff is easy to say right after an event happens... but let's be honest.
There's inherently nothing wrong with a high-school student wearing a trench coat. I can think of three or four that wore one in my high school. I can also think of people that didn't wear one probably being higher risk than those that did (very nice people, too.)
I don't think clothing is a big of a red flag as people want it to be. That's not what we should profile.