Again, it hasn't been a problem for the states where teachers are already allowed to carry, so I don't see how it magically turns into a problem if teachers being allowed to carry was expanded to more states. Obviously if you increase the number of teachers carrying, you increase the chance of some sort of incident, but 13 states isn't exactly a small sample size, so there is absolutely nothing to indicate that it wouldn't be an incredibly rare/non-existent problem even if the entire country allowed teachers to carry.
Now certain states are probably far too lax in their CCW licensing requirements, like Florida, but that's a separate issue. With Illinois finally being forced to join the rest of the nation in permitting CCW, I'd really like to see a reasonably strict federal standard for CCW licenses, and for it to be illegal for individual states to impose stricter requirements of their own.
Why are we even talking about this? Why the hell would a teacher be drawing on one of their students who didn't already have a gun? Also I'm pretty sure the number of people suicidal/heroic/stupid enough to charge someone with a gun while they themselves are unarmed is very close to zero, otherwise the various mass shootings of the past several years probably would have had a lower body count.