Well your professor is kind of a prick.
Both British and American English are perfectly valid ways to speak (or write )and if you use British grammer in your writing (I'm assuming your American here) than that is just your style, and he should just learn to deal with it. But unfortuantely some college professors can be real hardasses (yours sounds like this type of guy) and he probably won't listen to arguments or tolerate your writing with British English, so for the sake of your grades you may want to profread your papers ahead of time to fit into his prefered style. But still what a prick.
Both British and American English are perfectly valid ways to speak (or write )and if you use British grammer in your writing (I'm assuming your American here) than that is just your style, and he should just learn to deal with it. But unfortuantely some college professors can be real hardasses (yours sounds like this type of guy) and he probably won't listen to arguments or tolerate your writing with British English, so for the sake of your grades you may want to profread your papers ahead of time to fit into his prefered style. But still what a prick.