Grade grades mark marks

Dear teachers,

Would you be so kind to explain each difference

hidden in these three pairs?

(Are the differences only about the number of the subject?

means if the subject in the context is only one, then singular is used,

and if the subject in the context is more than one, then plural is used....

like this?)

(The first sentence is actually written in my workbook,

and the second one is what I thought could be possible.)

It seems that his grades have risen.

It seems that his grade has risen.

Study hard, and you will get a better mark.

Study hard, and you will get better marks.

Not only you but also Bob always gets full marks.

Not only you but also Bob always gets a full mark.

Warmest regards,

blissfarm