Skulls do not appear on Classic Mode.
However surprising and shocking it may be, this is true
.Sorry for the vagueness, I was thinking more about the effects of the skull, and if they would transfer over to Classic mode. Turns out they do.
But you can go directly above the skull in Classic mode and, by switching to Enhanced, stand on top of the skull. I think that you can walk around directly above the skull in the pickup radius without actually picking it up. Once you leave the pickup radius (and the skull) and go back to it, you will pick up the skull.