This article is from the Stand-up Comedy FAQ, by Steven Rosenthal and Steve Silberberg (firstname.lastname@example.org) with numerous contributions by others.
A. No one can teach you how to be funny. However you can be taught to be
less nervous on stage. You can also be taught some body language, how to
handle the mike, and how to phrase jokes. Unfortunately, you can also be
taught a "formula" for jokes.
Comedy classes are useful for building confidence, and sometimes getting
showcases and gigs. However, if you're not funny, comedy classes won't make
you funny. And they can be expensive.
One of the selling points of a comedy class is that your class is a built
in support mechanism. Improv works on this concept. So do jockstraps.
So the answer is yes, comedy classes are useful, especially if you're the
teacher. If you're a student, it depends.