A short while ago, we put up a piece on working towards a four year degree without paying for four years at a university. Here's some more ideas.
Besides advanced placement testing to "test out" of taking classes whose knowledge base you already have, at many high schools, you may have the option of "dual enrollment." A college or university may offer college classes at a high school, taught by high school teachers, for which students can get college credit. You might want to ask your local school if they offer this. For a look at this and a comparison with advanced placement, see this article from Princeton Review.
Another possibility aimed at disadvantaged students--those who are first-time college attendees or minorities--is early college. There are over 240 in the United States. These institutions are kept deliberately small to give extra attention to the students, and they are designed to increase the chance for these students to complete college by creating a challenging curriculum over four years that will end with students obtaining an associate's degree, and well on their way to a four year degree. Your chances of getting in are limited because classes are so small--only 400 students for the whole school. But if you fall into a group of students that your local early college targets, it might be worth your while.