Brothers have hidden twelve packages containing fireworks in Hogwarts. Here are their locations.
Note to number 1: look for it outside the tower, on one of shelves around it - the bird will knock it down to the ground.
Note to Hagrid's Hut neighborhood: most of the packages in this area are hidden in plants. Cast Accio on them.
Note to number 10: you'll probably find this package during Colin Creevey's task. It's on one of shelves.
Note to number 12: the package is behind the bookshelf in first room of the library - move it with Depulso.