Professor Lupine was having a terrible morning, and it wasn't even breakfast yet. Someone broke into the Charms classroom the night before, hid a Boggart in the closet, and set loose  doxies. How did he know there were 83 doxies? Because each one took a bite out of his nose as he tried to capture it, and Madame Pomfrey had to skip each wound separately. And comment annoyingly about how Professor Hagrid would've had the room cleared out with 2 waves of a wand.
It took forever to get out of the infirmary, and by then Professor Lupine was in a foul mood. Luckily, he knew how to improve it: by finding the culprits and making him, her or them pay. Lupine had overheard some students giggling in the hallways the other day about Lavender Ollivander's stash of doxy eggs, so he had a good idea of who to interrogate first. Unfortunately for him, but rather fortunately for Lav O, the common room password had recently been changed to Gryffindor, so Lupine couldn't enter. Former headmasters looked up from their portrait and laughed at the cursing, frustrated Professor.
The old Professor Lupine would have thrown a hissy fit at being taunted by a mere portrait, and blasted the thing all the way into the 5th floor girl's bathroom. The new Professor Lupine went to 15 weeks of Anger Management classes, punched a lot of pillows, and took up crochet. And started subscribing to the Weasley's Wizarding Wheezes catalogue.
Professor Lupine's first class off the day was Charms with the 3rd year Hufflepuffs and Slytherin. It took them 2 hours to clean up after the entire carton of Wildfire Whiz-Bangs that went off and clung to their robes, schoolbooks, and any exposed flesh. The students all knew that Lav O was planning to do something in Charms to get Professor Lupine, so everyone blamed her for the prank and gave her the cold shoulder for 17 days, until Lavender A blew up the Flying classroom with a misplaced charm and became the new pariah-of-the-hour.