I can think of several: Paducah, KY, that law school in NY, outside that courthouse in TX, and that's in about 30 seconds of thinking.
And the one that the MSM and gun grabbers really don’t want you to remember, the 1998 shootings in Pearl, MS - which were stopped only because the vice principal had a 1911 in his truck and was able to retrieve it:
http://www.davekopel.com/2A/OthWr/principal&gun.htm
The shooter in Paducah, KY, was arrested after he surrendered to the principal -- who had picked up the shooter's gun after he put it down.
The shooter in Springfield, Oregon was "taken into custody," by police -- he was not subdued by armed civilians or school personnel.
I can't recall enough to find specifics based on "that law school in NY" or "that courthouse in TX," but I don't seem to recall that armed civilians were involved in stopping them.
Not that I'm against law-abiding civilians carrying weapons -- I'm all for it, in fact. But the cases in question don't support the original claim.