So, essentially, you'd already have to have a concealed carry permit. Not sure how it works in S. Carolina, but here in WV it means you have to have taken training in firearms safety before being issued the permit.

For the record - I'd carry.

Not kidding.
They'd have to have every teacher carry otherwise what's the point? If one teacher in an entire school had a concealed weapon but was on the other side of the building when an attack occurred, they wouldn't have been able to do anything about it.