They're not clips, they're pop-rivets. You'll need a drill with a drill bit about the same size (but not smaller) as the hole in the centre and drill right through them. There's about 4 or 5 along the bottom of your sill. Then there's a small screw at the end of the skirt inside the rear wheel arch which is easy to remove and spot. If these are the original Sport skirts, and have never been removed, then there's going to also be about 6 black rectangles which hold the skirt up in place pop-riveted along your rear quarter and along your sill. Your 172 is going to look hellishly stupid with those still on and no skirts. If you want rid of them, the holes on the bodywork will have to be filled and painted.
Enjoy.