You make several assumptions that reveal your ignorance.
First you assume that a school administrator that has taken the responsibility of protecting the children under hir or her charge has no training, nor have they practiced. This is quite a stupid assumption.
Second, you assume that the gunman is an expert, forgetting the fact that he has chosen a school specifically for the reason that schools are gun free zones. Imagine his surprise and terror when facing an unexpected armed response.
You should be embarrassed, because your complete ignorance makes you a joke.