What are some good Harry Potter room decorating ideas?

Question:For the last week of school, several groups of students are volunteering to decorate empty classrooms with different themes. We charge money after school one night for everyone to visit them and the money proceeds to an end-of-the-year celebration for all grades. Me and my friends (4 of us) decided to go with a Harry Potter theme. We're each gonna dress like 1 of the 4 houses, but we're lacking in ideas.
Any suggestions?

Answers:
ideas here for a haunted mansion, harry potter
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