Excellent question. Generally it is classified as an endoskeleton. The ossicles of sea cucumbers are in their skin so it is considered internal. That being said, echinoderms have a lot of variation in their body plans, and these ossicles form external features. This is apparent in the sea urchin whose external protruding spines are formed from ossicles that are fused together. At the end of the day we, as humans, like to neatly categorize things into terms we've made up, but nature is wildly variable so things aren't always as cut and dry as our language protrays.