Plurals of acronyms, abbreviations, initialisms and single letters do not usually take apostrophes. Guidance on this issue is not unanimous, but some general advice can be consistently applied, and may help resolve some of the widespread confusion that seems to be generating even more widespread confusion.
Pictured is a screenshot from Twitter, taken a few months ago, showing popular subjects or “Trending Topics”. The vast numbers of people using Twitter means that for a subject to become a “Trending Topic”, it needs to be mentioned a great deal. Because the software distinguishes between two words that are identical apart from the presence or absence of an apostrophe, both “SATs” and “SAT’s” have the potential to appear concurrently in the list, as indeed they did. Evidently, both forms are widely used. My preference is for “SATs”, and I would consider “SAT’s” ill-advised, because such apostrophes are largely unnecessary and potentially confusing.
A recent report from TechCrunch illustrates the potential for miscues from apostrophised plurals: