One of the options of alter self doesn't specify creature type when transforming appearance.
Change Appearance. You transform your appearance. You decide what you look like, including your height, weight, facial features, sound of your voice, hair length, coloration, and distinguishing characteristics, if any. You can make yourself appear as a member of another race, though none of your statistics change. You also don't appear as a creature of a different size than you, and your basic shape stays the same, if you're bipedal, you can't use this spell to become quadrupedal, for instance.
Would it be reasonable to assume that assuming the form of a Skeleton is allowed?