You deserved to be kicked out. Many, many school shooting start with similar jokes about "bringing a gun to school lol!" Example: http://abcnews.go.com/US/story?id=93924&page=1.

All I see in this thread are a bunch of whining millennials bickering over policy details. How about: Don't joke about killing people at a public event? Your words and social media posts are your public record and indicate your intent, don't post stupid things online, learn that now before you do something really dumb that gets you fired.