If you keep the periods (full stops) then it's acceptable to use an apostrophe "s", (otherwise it looks awkward): M.A.'s
If you don't use periods, just add "s": MAs
It is increasingly common and considered improved style to drop the periods from abbreviations.
I don't think it's AE vs. BE. I'm a native of AE, and I would never put an apostrophe unless I wanted to indicate the possessive.This is probably an AE vs. BE thing, but I think you should put the apostrophe regardless of whether or not there are periods (M.A.'s or MA's).
Adding the apostrophe is the more traditional practice--the one I was taught as a student--and is used by the conservative (in language style) New York Times, among others. See, for example, the use of "CD's" and "DVD's" here.I don't think it's AE vs. BE. I'm a native of AE, and I would never put an apostrophe unless I wanted to indicate the possessive.
The CEO's speech was well-received.
The CEOs all had lunch together.