Re: got top marks/got a top mark
I think that "Cassie got top marks" is ambiguous. It could mean either shat she got a higher mark than anybody else in the class, or that she was awarded the highest mark possible.
I don't think that 'a top mark' is natural English. 'A good mark' and 'good marks' are both possible. They may have very similar meanings, though the singular form could imply 'a good overall mark/grade/' and the plural 'a good mark in each section of the paper'.
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