All sugary and fizzy drinks attack teeth because of the acid in them. If you leave a tooth in a glass of cola overnight it doesnt disolve, but it will discolour. You could disolve a tooth (or even a skull) eventually, but it would take a long time and a constant running supply of drink.
This is a bit of a myth in the sense that its strongly exaggerated. Drinks like coke or pepsi are indeed acidic and can cause corrosion and degrade stuff, so yes, it would, but it would be probably less dramatic than you might expect. By the way, why do you want to degrade a skull?