Quite possibly, a locked door and no signage. The killers may not even have been aware of the school depending on when they scouted the store.
Two cops engaged them within a minute of their attacking store.
Whatever the reason this carnage could have been worse than the horror it was.
Did they carry so much ammo for a shootout with police? Or expecting more victims than could be found in a local grocery? They said the grocery was targeted.