The fact that Skeleton tokens are destroyed is an aspect of the enchantment and not of the tokens. Thus, only Skeleton tokens which are on the battlefield when it leaves the battlefield are destroyed. Ones which are going to appear due to use of the ability but which have not yet appeared are safe.
The creatures in the graveyard are exiled during activation as a cost.
This destroys all Skeleton tokens from all sources if it leaves the battlefield. It does not just destroy ones generated by this Drudge Spell or just ones you control.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. NéNé Thomas
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