Screw Java. They can learn syntax on their own. Teach them Smalltalk. Class hierarchies are a small-minded hack to provide static type-checking. You don't bother with static type-checking in your first teaching language (Scheme), so why bother in your second? Java is a practical language, but not a good teaching language. Smalltalk, however, is as purely object-oriented as Scheme is functional. It also gives you first-class functions and such, like Scheme.
Teach some Java at the end of the course if you have to. But teach them object-oriented programming with a properly object-oriented language. (Note: there is a GNU Smalltalk environment.)
I have difficulty enumerating the many, many ways in which this suggestion fails to be at all useful. Already, I can't start the class in Scheme; why do you think I'd be able to add Smalltalk? You imply that because my first-language-of-choice doesn't have static types, the second needn't; indeed, why ever add anything at all? I'm not sure if the comment about hierarchies is just a diss on type inheritance or on the entire concept of type checking, but either one's going to require a lot more backup than just a bald statement like that if you're going to try to convince me. In any case, Smalltalk isn't as purely OO as you might think. Self, now Self is OO.