University-level writing assignments should not contain spelling errors. Proofread your work carefully, consulting a good dictionary (e.g., Oxford Canadian Dictionary).
Spell-check programs will not identify homonym errors (it’s/its, than/then, lead/led).
Note that you should not alter the spelling of your sources when quoting; see QP.
Make sure that your spell-checker does not automatically change words you do not want changed (such as “Puck” to “puke”).
Canadian, British, and American spelling conventions differ. For example, this website uses Canadian spelling conventions (“theatre” rather than “theater,” “colour” rather than “color”; either “realize” or “realise” is acceptable in Canadian spelling, although you should be consistent in your choice of z or s in all such words). Whichever you choose, be consistent and check the appropriate dictionary. Note that most spell-checkers set the U.S. dictionary as the default; if you opt for Canadian spellings, you will have to select the dictionary manually (from the Tools menu in MS Word).