I haven't heard of any special Harry Potter 'do' being organised around here, but 2 years ago school had a Harry Potter day and a disco/party in the evening.
For day wear I assembled -
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And
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Daddy didn't have any striped ties, but I had a 60" x6-7" scrap of stripy fabric, which I cut into bias strips and re-assembled to make her tie - the fact that the stripes also matched was pure luck, given the amount I was working with.
Then for the evening party -