Differentiate between Present Simple vs Present Progressive (Continuous)

I am in class - is a declarative clause in the Present Simple.

Subject= I

verb= am acts here as a linking verb or copula, and thus the clause's verb

your complement - in class(might be an adjunct, not too sure) doesn't allow for the sentence to be said in Present Progressive.

I am being in class does not work because you need a "dynamic adjective" as a complement for it to work.

as in:

I am being nice, or I am being funny.

Nice and funny are adjectives that can complement be in Present Progressive ( there aren't too many of those).