As a college freshman, here's my view on high school work: just do it. It's the only way to get into college. That has to be your sole motivation, as for most intelligent people, high school work in itself is not interesting in the slightest (although I was lucky to take a Constitutional Law class senior year that I loved).

However, don't despair: college work IS interesting. Unlike high school, a college will let you take essentially whatever class you want. Interested in physics? Take a class on special relativity. Interested in philosophy? Take a class that applies the theories of the great philosophers to modern moral problems. Interested in spaceflight? Take intro to aerospace engineering. At a university, the possibilities are endless, especially freshman year when you don't have to worry in the slightest about your major.

So, to sum it up, this is my advice: high school work sucks, but college, at least from what I've found, is awesome. Not only are you encouraged to take classes in areas that interest you, the atmosphere and social life is also incomparably better than high school. Your motivation thus should be directly tied to college: if you do not do this paper tonight, you will not get a good grade in this class, and you will not be able to get into as good of a college, meaning you may not be able to take high-quality classes in the areas that really interest you. It may seem like negative motivation, but if you view college as something good for you (and not just good for your career), it really becomes positive motivation. Good luck.