Not really sure where to draw the line. I imagine there is a point when you start to get to the line where things start turning gray. As I recall, when Rosa Parks passed away the flags were lowered. I agree with that although I am sure there are those who for various reasons aside from race would say it was not warranted. I would not argue that point. I would strongly argue that lowering for the VT kids was not warranted. They were in the wrong place at the wrong time. They do not warrant a flag at half staff any more than any other murder victim.
I read a blog/article a few days ago (can remember where) but the author made the point that he did not feel that lowering the flag for fallen soldiers was warranted either. The reasoning being is that any fallen soldier receives honors not granted to any others such as color guard, burial at Arlington and the like. He was not saying that the few states who do lower the flag for fallen soldiers from their own state was wrong (in fact he thought that was a very good gesture) he just argued that it should not be a Federal mandate.
I tend to side with the old tradition. The flag should be lowered for heads of state and high ranking dignitaries in the US government. Of course there will be exceptions but they should be carefully thought out. Lowering the flag for the victims at VT in my opinion was wrong and an insult to those who the flag should be lowered for.
Just my opinion.