As we all hopefully know Harry Potter is set in Britain and we Britons spell things differently from you Americans. Not much differently just ever so slightly differently. And with Hogwarts being British they speak and spell things the British way. They do not call their mothers 'mom' they would call her mum. Now obviously if you're American or come from somewhere where they spell things differently from the traditional way that's fine but it would just make your story that bit more realistic and believable if you spelt things 'correctly'. I've made a list of some of the words we all spell or say differently, American on the left British on the right.
Mom-mum
Color-colour
Chips-crisps
French fries-chips
Candy-sweets
Center-centre
Apologize-apologise
Traveled-travelled
Defense-defence
Catalog-catalogue
You get the idea? It just really helps your story to be more Harry Potter-y if it's spelled the way it would be spelt in the books. And I know that it is very easy to make mistakes but that's fine even if you spell one or two words the British way it still shows that your trying.