Why was an armed person allowed to enter the Robb Elementary School unchallenged?

[Klem]

The sequence of events, particularly the timing of the door being propped open and apparent police incompetence/cowardice, are leading some to hint if not outright speculate about conspiracy.

Occam's razor suggests it was just a door that was propped open to facilitate school business. Coupled with bureaucratic hesitancy on the part of first responders, perhaps even a lack of proactive leadership. Speculating it was a deliberate higher conspiracy to further some ulterior agenda is the stuff of fiction.