Most skunks are poor climbers and are usually unable to climb fences but it is entirely possible for some skunks to climb fences.
I've personally seen a skunk struggle to climb a fence in my yard and the skunk did eventually make it up and over the fence even though it was a struggle for the skunk to climb the fence.
So while most skunks cannot make it up and over a fence it is possible although most skunks will not even try to climb fences.
Skunks are also fantastic diggers so they can tunnel their way under your fence as well.
The raccoons and possums are better climbers than skunks and can easily scale a fence while the skunks can climb fences but it's not as easy for them to do so.