A reply to the original post by @nicksimard. This is the scenario I envisage:

Active shooter, the school is in lockdown. In Miss Libney's class they've already bolted the door. John - it had to be John - begins to whine softly to himself. Rachel glances over, sympathetic, "We'll be fine, John, we'll be fine, don't worry". She puts a hand on his shoulder and rubs. "Don't worry you'll see, it won-"

The smell hits her then, pugnacious, like a one-two punch to her olfactory receptors. It's foul, rank, like last week's lunch left inside of a classroom bench, if the lunch was rotten egg salad and a decaying seagull. It's the sickly sweet part of the stench that gets her; it connects with an ancient part of her brain, one that knows of pestilence and death. She retches and backs away quickly, gulping fresh air - hoping there is, still, such thing as fresh air.

By now most of the other students have detected John's opulent flatulence.
"Ewww, gross!"
"Aww man, did you eat a shit?!"
"What the fuck is that? What the fuuughhh!"
"Miss, can we just leave?"
At this point half of the class is groaning and carrying on as if they'd been shot. Irony of ironies, the active stench takes their minds off the active shooter. John, in his personal hell, is clutching his stomach with both hands, hoping that if he maintains the pressure on his gurgling insides he can keep the dam from bursting into an effluvian sea. He's cursing the lunch ladies, a part of him hoping the gunman is carrying a grudge with their names on it.

Miss Libney tries to quiet everyone down, "Hey, hey, hey!! John, or whoever did that, the emergency bucket is in the corner, go use it, now!"
Everyone is holding their noses and looking at John, everyone knows he is the malodorous maladroit. Right now, at this very moment, he is reviled by his peers more than any unseen and unknown gunman could be. Averting his gaze from the angry eyes around him, hoping it's just a nightmare within a nightmare (it must be, it must be, he thinks), he shuffles to the corner where Ms. Libney, reacting as trained, had removed the EmergencyLockdownBucket™(ELB sold by AMCO Plastics at $699 per unit) from the cramped utility closet right after bolting the door.

The pain is so severe and the relief at hand so enticing he doesn't even care anymore. Facing the class he unbuckles, he sits, and he shits.

Now John is no longer in the classroom, he's drifting away, wave after wave, like the lyrics in that Robin Schultz song, expunging the contents of his large intestine into the bright yellow ELB, becoming more lightheaded, more euphoric, more in tune with his inner self at every spasm of his colon. His classmates' wails of despair, which harmonize with the resounding squelch of diarrhea hitting the bucket, no longer register. His eyes are closed in contentment, his body feels lighter by the second, the relief is overwhelming. Then a series of dull thuds snap him out of it; somebody is trying to smash in the door. The wails of disgust turn into screams of terror.

That's when John realizes there is no emergency toilet paper (ETP sold separately at $88 per roll). That's when he realizes, much earlier than most people do, that this was his destiny all along.

Standing quickly, and now indifferent to wiping, John buckles his belt and, fixing his cowering colleagues with a look of hard-nosed resolve, grasps the bucket by the handle. As the ensuing hail of high-caliber bullets shatter the lock and send the bolt flying across the room, there is a moment of such serene calm, and the quiet weighs so heavily, that Rachel, now holding on to Alice, believes, as if in a Borgesian fabulation, that this is a miracle; time might have stopped and the world has frozen still.

This illusion is shattered by the appearance of a high-lace, platformed boot smashing into the door with such ferocity forensics officers will later find splinters lodged into the wall behind. The students barely register the bloody footprint it leaves in the wood-grain when a maddened voice yells out, "KNOCK, KNOCK, BITCHES!"

The gunman, clad in the dripping red attire of carnage, his eyes bugged out in a world annihilating blood-frenzy, takes a step inside, bringing the automatic rifle to bear, when a full gallon of John's still warm, pathogen-laden feculence rains down on him like ten biblical plagues rolled into one. He is blinded, unable to breathe, and loses his balance on the slippery sludge around him. He chokes, engulfed in the cloying vapors of his destruction, and drops his weapon.

Swinging the ELB by its handle and connecting with the shooter's poop-dripping jaw, John sends him crumpling into a puddle of refuse so foul-smelling a latrine would be offended. Kicking the excrement-coated rifle away, he looks down to the gagging, human sized shitstain at his feet.
"Eat shit, motherfucker." John says.

---------------------

Weeks later, standing in front of the President as recipient of the Presidential Medal of Freedom, the nation's chief executive asks John how he did it.
Beaming with pride, John says simply, "I was only doing my duty."