I'm not sure that there is a grammatical explanation for it. The difference is regional. You would use the first in the United States and the second in the UK. Can any BE speakers explain why "full marks" is plural?
As to your question, we get grades, rather than marks. The usage of the word seems to be a little different, too. You wouldn't get full grades for anything. You would get a perfect score or a full score. Then, your grades in the class determine what grade you get for the semester.
One can (or could when I was at school) get good marks on just one exam!I wasn't aware that this was a BE term - do you not get marks on tests, etc. in the U.S.? In Canada, I might write an English exam where my score is 89% and my mark for the semester is a "B" in the English course (as a result of my participation in class and having taken many tests and quizzes). Or my score on every exam might be 15% and I get a mark of "F" (fail) in the course for that semester. My marks for each semester will determine my final grade for the year.
As to the original question, a score is singular because it's one score (the result of one exam) and marks are plural because one "gets good marks in the course".
I would never say I got perfect marks, such a thing being impossible to my way of thinking. 'I got very good/high marks' would I think be the more usual expression.
Another difference: In American English, we don't write exames; we take them.In Canada, I might write an English exam where my score is 89%...
Is it only Canadian English that you can get "a mark of..." in one test?Or my score on every exam might be 15% and I get a mark of "F" (fail) in the course for that semester. My marks for each semester will determine my final grade for the year.
Is it only Canadian English that you can get "a mark of..." in one test?
get a high mark in a test
get high marks in a test
I've heard that phrase too, and the context, as I understood it, was "perfect score on that last round", switching to AE.I have heard the expression "full marks" from the host on the old BBC Radio shows "My Word" and "My Music". "Full marks for Ian on that last round!" If it doesn't currently exist in British English, it did in the recent past.