I can see the teacher's perspective here. I don't think it's unreasonable for teachers to mark this down, though I don't think your request to get back the points was unreasonable either.
Communicating effectively with your particular audience (the local American audience one in this case) is arguably a skill they're trying to teach you. Using language constructs that are unnatural for your audience can distract them and disrupt their flow. You don't want your reader to see "enrol" and spend the next 5-10 seconds pondering if you're illiterate or unable to run a spellchecker instead of spending that time digesting your actual point. This friction gets in the way of your point, thus ultimately hurting your own cause in addition to wasting their time. So when you keep doing that on your assignments and exams, you're effectively showing that you haven't mastered this skill -- and so it's not unreasonable for your grade to reflect that.