Hi there! As others have mentioned, PbtA games give players a lot of agency to co-create the scenario with the GM. Note the game instructions about asking the players when you don't know something. The GM is not the ultimate font of all knowledge in this style of game.
My favorite trick is to start the game by asking each player about one rumor they've heard at Hogwarts this year. Then, use one or more of the rumors to introduce your initial unexpected event. As the players investigate what's going on, they'll start seeing mysteries to solve. And as you introduce more events based on their rumors, you'll all start to see interesting connections between them – that's the scenario.
If you're still stumped, email me at david@davidbrunellbrutman.com. I don't have any public-facing pre-written scenarios, but I can share my personal notes for the scenarios I run at conventions.