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As Casper Weinberger used to say, it’s a known unknown.

There is no central tally of the effects, with states often barring release of concealed-carry data...

source: https://www.nytimes.com/2015/02/11/opinion/concealed-carrys-body-count.html

So where does the homicide data come from?

Often and always aren’t the same.

Meh. If something like this can happen with a police officer in a school, what are the odds that teachers with guns won’t lead to similar incidents multiplied?

http://minnesota.cbslocal.com/2018/02/06/3rd-grader-fires-gun-maplewood/

I know they aren't the same but where does the data come from?

Court records I would presume. Or maybe news stories like this one:
http://blog.al.com/spotnews/2009/12/pharmacist_charged_in_officers.html