From Wikipedia: "
Terrorism is, in its broadest sense, the use or threatened use of
violence (
terror) in order to achieve a political, religious, or ideological aim."
The school shooters were responding to those who they believed had different ideologies.
Also from Wikipedia article about Columbine: "The attack has been referred to by
USA Today as a "suicidal attack [that was] planned as a grand—if badly implemented—terrorist bombing."
If we can't agree that school shootings are acts of terrorism, I can't imagine there is any further point in continuing this discussion.