Harris and Klebold attacked a segment of a civilian population who they believed had wronged them with the purpose of intimidating that population, creating fear, and bringing about social change.
Unless you have some evidence documenting that these guys were trying to effect social change, their shooting doesn't go beyond simple revenge. Change may have resulted, but there isn't any indication that social change was one of their goals, at least not of which I am aware.
And, semantics are critical. Calling all mass shootings and such terrorist acts is a sensationalization that adds attributes to the events that are not present. This is something we like to harp on the media for doing whenever they make more of a shooting than exists or imbue attributes to the shooting that were not present, such as the association of guns with being bad or evil.