Dementors never rejoined the ministry. They just waited for a new dark wizard of whom they could seek their allegance with. Then they decided they could fight it on their own. So they went after Hogwarts and possibly bring revival to old evils. To name some the Basilisk, Cornish pixies, dragons, merpeople, giant spiders, trolls and much more evil. So they attacked in september.
Elizabeth is sitting in her common room studying transfiguration when she feels an awful chill down her spine. She jumps up and runs to the front gates of Hogwarts. A deep fog is setting in and Elizabeth has a bad feeling about this. She draws her wand as the fog is getting thicker and thicker.
Tom was up on the Owlery, preparing to send a letter back to his family with his owl. As he releases Speckley the Owl, he sees a thick fog racing over from the distance towards Hogwarts. He pulled his cloak in a little tighter, it was starting to get a little chilly, best to get inside.
Hermione in the Ravenclaw girl's dormitory when everything went dark. She had a feeling of hopelessness in her. "This must be the dementor," she thought."EXPECTO PATRONUM!"she bellowed, thinking of a happy memory. A little silvery fog came out from her wand. "EXPECTO PATRONUM!" she tried again, thinking of a happier and stronger memory than before. A large unicorn erupted from her wand. It charged at the dementors to drive them away.
Elizabeth was still outside by the front gates but she can here screaming from inside. She held her wand at the ready as that yellow light persistantly approached. Elizabeth slowly backed away from it back towards the school. She needed to get help.