Professor Remus Lupin was having a terrible morning, and it wasn't even breakfast yet. Someone broke into the Muggle Studies classroom the night before, hid a Boggart in the closet, and set loose 613 doxies. How did he know there were 613 doxies? Because each one took a bite out of his elbow as he tried to capture it, and Madame Pomfrey had to sleep each wound separately. And comment annoyingly about how Professor Horace Slughorn 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 Remus Lupin was in a foul mood. Luckily, he knew how to improve it: by finding the culprits and making him, her or them pay. Remus Lupin had overheard some students giggling in the hallways the other day about Antonio dela Weasley's stash of doxy eggs, so he had a good idea of who to interrogate first. Unfortunately for him, but rather fortunately for Antonio dela Weasley, the common room password had recently been changed to Chudley Cannons, so Remus Lupin couldn't enter. The Pink Lady looked up from her portrait and laughed at the cursing, frustrated Professor.
The old Professor Remus Lupin would have thrown a hissy fit at being taunted by a mere portrait, and blasted the thing all the way into the 6th floor girl's bathroom. The new Professor Remus Lupin went to 14 weeks of Anger Management classes, punched a lot of pillows, and took up Basket Weaving. And started subscribing to the Weasley's Wizarding Wheezes catalogue.
Professor Remus Lupin's first class of the day was Muggle Studies with the 3rd year Hufflepuffs and Ravenclaws. It took them 2 hours to clean up after the entire carton of Portable Swamps that went off and clung to their robes, schoolbooks, and any exposed flesh. The students all knew that Antonio dela Weasley was planning to do something in Muggle Studies to get Professor Remus Lupin, so everyone blamed him for the prank and gave him the cold shoulder for 17 days, until Ursula Longbottom blew up the Flying classroom with a misplaced charm and became the new pariah-of-the-hour.