BrE vs. AmE
I so admire anyone who can communicate in English as their second language that to me minor variations in AmE vs. BrE don't matter a bit. I'll note them for the students in the margin and leave it at that, but certainly not penalize students for using one or the other when both work just as well for communication. If for some reason a writer will be looked down upon or taken less seriously for mixing AmE and BrE, that should be discussed with the student, who can then decide how important it is to him to learn the distinctions and abide by them in his own usage. But I do not believe students should be penalized in the language classroom for using one instead of the other. Informed yes, penalized no.