The apostrophe indicates the plural of the letter "A". It is not the possessive. There are some style guides that say that the apostrophe is used only to make the plural of lowercase letters and literal words.
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Hariss was so happy to learn that he earned 2 As and 3 Bs for this semester's classes
Strunk and White say the only time you use the apostrophe to pluralize are in cases of the p's and q's mentioned above, or acronyms that have both upper and lowercase letters so as to avoid confusion.
Straight As
1960s
dot your i's
NiMH's