Harry: Minnie and Mickey Mouse
Ron: Grapes (idk if you guys have seen the grape Halloween costume made out of balloons but that's what I'm picturing)
Draco: Lola and Bugs Bunny
Fred: The Joker and Harley Quinn
George: Sandy Dee and Danny Zuko
Oliver: Your favorite quidditch players
Seamus: Pirate and mermaid
Dean: Gomez and Morticia Addams
Neville: Peter Pan and Wendy