Construction of S-box in AES
In the field $GF(2^8)$, $x^{254} = x^{-1}$ (except for $x=0$, as $0^{-1}$ doesn't exist; for AES, that's treated as 0), and so it's two ways of describing the same thing.

When we talk about AES, we typically use the $x^{-1}$ nomenclature; for whatever reason, your class decided to go with the $x^{254}$ one.