What's the difference between 'mark' and 'grade', in an educational context?

It's been suggested to me that "mark" carries a slight presumption of a numerical grade, whereas "grade" implies a letter grade like A, B, C, etc.

In American English, 'mark' is not commonly used to describe anything along these lines. At least, not in the West, Mid-west, or Alaska, where I have been. If someone said, "I got good marks in class," most everyone I know would either not understand the sentence or have to think about it for a moment. This may not hold true for New England, California, or other areas where I have no experience and the word may be more commonly used.

To better answer the question directly, if spoken in areas of the USA I have been in, the two would be seen as synonyms. The listener may assume the speaker is British, and that assumption may carry other connotations with it, but the words themselves would not be used differently nor carry different connotations.

In common American English usage, I think the two are synonyms. "My mark was a B+", "My grade was an 89" ... I don't think anyone would find those statements puzzling.

In British usage, a marking scheme is usually drawn up for an exam or test. The completed papers are marked (the mark being a number, out of perhaps 100 marks available). Ignoring moderation (where the test itself is judged and grades and possibly even marks are adjusted), perhaps the top 10% of candidates are awarded an A grade, the next 20% a B, the next 30% a C, the next 20% a D, and the rest an E or an F.

Marks is used commonly in Canada. Marks is actually used more than "grade" or "score". It's common to hear kids ask, "What mark did you get?"