The school shooters usually show a pattern of feeling on the outs in their school and many times have suffered from bullying, esp. homophobic taunting. Thus, they have a revenge motivation and part of that is to show the taunters that they can't get away with it. But that isn't to influence a major societal institution so it changes policies as directly as most acknowledged terrorist acts.
Also, the school shooters usually show some history of psychopathology. Despite some rhetoric most organized terrorists don't have such histories.