but he was engaged outside of the building by an SRO, who was shot. the kid dropped his duffel bag containing extra magazines. one way to mitigate these things is access control to the building
A news story had it that the killer gained access via an outside door to an empty classroom. That door would have been confirmed locked by the custodial staff every evening, meaning it was left unlocked or slightly open that school day either for convenience or by oversight.
"...if the gentlemen of Virginia shall send us a dozen of their sons, we would take great care in their education, instruct them in all we know, and make men of them." Canasatego 1744
