Gun man comes into a class - armed qualified teacher engages. Will 20 or 30 kids be hit by friendly fire. Probably not. Let's say 2 get hit (a tragedy), 28 are saved. The moral onus is on the gun man and not the teacher that tried his or her best. That is especially the case, if the latter took the time to train and made a sincere effort to be prepared.
Not to argue both sides, but I think that he was talking about an accidental (or negligent) discharge, rather than collateral damage in an actual defensive situation.
Regarding gun accidents in public, they are extremely rare - but not nonexistant - even if we just look at the public at large. That includes several shall-issue states where you can carry or get a concealed carry license with NO training whatsoever. Off the top of my head, these include Washington, Indiana, Pennsylvania, Vermont, Arizona, Alaska - and probably others. We just don't see many accidents in public or gun grabs.
If we used this as a starting point, and then asked whether there were some
"reasonable" training requirements that could/should be imposed, I'm with you.