You have likely been in many other classes up to now. The classes that involved writing in English had teachers who taught you the preferred English style for the country you were in.
Canada has its own writing idiosyncrasies. We like our letter zed. We like it in many places where other countries like a soft, sibilant "s." Not us. We want to IZE everything.
Look in the dictionary. The word "realise" is not there. Don't apologise for not checking. Apologize! I will never sympathise with you for your pain. But I might sympathize.
In Canada, you will have to prioritize, accessorize, empathize, and finalize. Here are some exceptions:
You can look here for an explanation of this. As the linked article says, English spelling is "untidy." I'd say downright messy. Buy and use the POCD if you intend to write Canadian English better (dammit).