Hi Yoshi,
The word
A's functions as a
plural noun. If we add the noun "grades", the result is ungrammatical:
*He got all
A's grades. (ungrammatical)
There are two problems with the above sentence:
1) if
A's is a noun, then there should be a conjunction joining it and the other plural noun (
A's is a plural noun and
grades is a plural noun.)
2) If
A's is an adjective, it shouldn't carry inflection. The letter "s" represent inflection. Only nouns inflect. Adjectives never inflect. Now, if we delete the infection marker, the sentence is fine:
He got all
A grades. (grammatical; adjective + noun)
The phrase "A grades" functions as a noun. It's made up of the adjective "A" and the noun "grades". The adjective "A" modifies the noun, telling us what kind of grades.
In short, "He got
all A's" is grammatical. "A's" is a plural noun.
As for the apostrophe -s, the -s represent plural number (i.e., "A's" means, more than one A), and the apostrophe is added for clarity. It doesn't express possession. Watch what happens when we omit the apostrophe:
A's => As
The word "As" is ambiguous; it has more than one meaning:
As much as I want all
As, I probably won't get
as many
as I hope.
The words in
bold and the
underlined word carry different meanings. So you see, apostrophe -s is added to some plural nouns, like "A's", to clear up ambiguity.
Closing Note, "A's" is not ambiguous in "He got all A's", but an apostrophe is required nonetheless for the sake of consistency. Use either the noun "As" or the noun "A's", not both.
All the best,