Perhaps the standard grading system would be A, B, C, D, and F (with + or - for each; e.g., A+, A, A-, B+, etc.) in the US (and probably also in the UK), so "A, B, C (or D)" may be closer to "よくできる, できる, もう少し."

Although it is not very common, I thought "excellent, good, unsatisfactory" would be clearer for the translation purpose, especially when there are only three grades and no additional grades that correspond to the rest (D and F).

"(there is) room for improvement" or "below average" may better explain "もう少し" if a longer phrase is allowed.

Academic grading in the United States
https://en.wikipedia.org/wiki/Academic_grading_in_the_United...

Academic grading in the United Kingdom
https://en.wikipedia.org/wiki/Academic_grading_in_the_United...

Grading System in the United Kingdom
https://www.scholaro.com/pro/Countries/united-kingdom/Gradin...