Why is it that you have to put an apostrophe for when you're talking about grades, like: A's?

If you don't, it will look like the word "As." The use of the apostrophe keeps the usage and meaning clear.

If you don't, it will look like the word "As." The use of the apostrophe keeps the usage and meaning clear.

Is there a specific name for it? Hope that made sense.

Is there a specific name for it? Hope that made sense.
No, there is no specific name for this. Clarity is what you want.

If you have more than one capitalized letter, such as CPA, and make it plural, you don't need an apostrophe --

CPAs (plural) -- CPA's (possessive). '
The company hired three CPAs.
The CPA's pen ran dry.

CEOs (plural) -- CEO's (possessive)
Two CEOs retired yesterday.
The CEO's purple Mercedes got a flat tire.

RNs (plural) -- RN's {possessive)
All the RNs wore floral smocks.
The RN's smock was covered with blood.