When I was a kid, Halloween had not yet fallen to political correctness. It was actually celebrated in schools through all-afternoon parties which kids attended in full costumes that included skeletons, vampires, and creatively-assorted monsters. I can remember going in a leopard costume one year (yes, I was a furry even back then), and as a robot another year. Even the teacher would usually dress in costume, often as a witch...and we had a blast!
...times are sadly different now. Religious fundamentalists have assaulted school Halloween observances as "promoting Satanism in the schools." You can't even have Halloween parties anymore...they are called "fall parties," and kids are encouraged to dress up as characters from childrens' literature...what fun!
Forget about Harry Potter, though. He's a literary figure kids love, but he's also a young wizard attending a school called Hogwarts where magic is taught, so he can't be referenced in schools, or the teacher would be suspended.
All in all, I'm glad I was of school age in a time before Halloween was sanitized!