I don’t think you can say either one is it isn’t universally. Terrorism is using threats or violence to achieve a political goal. If a kid goes to school with a gun and kills a teacher he doesn’t like, or if someone kills a CEO because the CEO is a bad person, that’s not terrorism. Intent matters for the definition. If, say, a CEO is killed to make a statement about CEOs or to get that kind of business, then it could be. It’s harder to see how the word applies to school shootings.