It only becomes a police issue when they become violent.  The reality is that young, isolated  males are genetically pre-disposed to commit this types of crimes.  Combine their genetic pre-disposition with societal influence and you get ticking time bombs - guns or no guns. 
Your call to make this about race but its not - it's about parenting, mental health, a shitty system...etc.  I could go on and on.  Gang violence is a different cup of tea.  Brown on brown, black on black violoence is anchored by socio economic conditions.  Walking into a classroom of young children and shooting them in cold blood and watching them bleed out (much faster for small bodies)  is different - the warning signs are usually there and they are usually ignored.  This will happen again, I have no doubt. 
Next time lets hope the school doors are locked, cops with balls show up, and someone takes charge.  Preferably there will be a system in place that provides enough of an obstacle that these types of assholes don't get inside of a school.  I dont' think that is too much to ask for.  As a test, go and try to gain access to a corporate building somewhere.  Most high value corporations spend millions of $$ on phyisical security.  Amazing what badge readers do to deter entry.