First of all; the last line should have read: This ‘ONLINE ESSAY’ will get you ‘GOOD GRADES’ and not ‘this online essay will get you good grade’
Second of all; if you need others to write ‘ESSAYS’ for you then you shouldn’t be where you are – wherever you are – whether college or university.
Third of all; the best rule to follow when writing – whether ESSAYS or whatever – is:
FOLLOW THE RULES AND YOU FORFEIT YOUR FEELINGS; and your ESSAYS or whatever may turn out to be good, but they’ll be dry.
My rules are:
1. WRITE WHAT YOU MEAN AND MEAN WHAT YOU WRITE
2. FOLLOW YOUR FEELINGS AND FORGET THE RULES
Then arrange or organize your feelings into paragraphs accordingly – as best you can.
The above (my) rules apply only to those that can write – at least

Grade means 'A mark given in a exam for A PIECE of school work.

we use.

She got good grades in her Exams.

She got good grade in Physics.

Should we say.

SHE GOT GOOD GRADES IN PHYSICS.