Note: I have finished a short story of the idea outlined below. I will publish it here later, if and only if there seems to be no hope of publication elsewhere, in small press journals where I have already started submission.
This is from a news piece about a man in his early eighties, who was volunteering at the library, running a small seminar on reading, tutoring some children...
A clearly deranged individual came into the room, brandishing two knives, and loudly proclaiming his intention of killing kids. This fat old man, ex-military from long ago, thought "Not on my watch" and set himself to defending his charges.
He was seriously injured; cut heavily with defensive cuts on his forearms, but he managed to subdue the attacker and sat on him while one of the kids ran to get a librarian to call 911...
Make this into a story. Try to include some dialog with his wife, who perhaps was due to pick him up to go back home, and interrupted the fight at its very end... Try to set up a touch about one of the kids who has tough issues and hates these sessions with the old guy-- maybe he is the only one who is unafraid enough to run for help...
Title of the story: Not on My Watch.