Like mentioned above, many UC campuses have open campus programs. Enrollment and transcripts may be handled by Extension, but you are enrolled in regular classes on campus. Here are UCSC and UCB’s programs:
http://www.ucsc-extension.edu/open-campus
http://extension.berkeley.edu/static/studentservices/concurrent/
Open university is also available at both SJSU and CSU East Bay. Santa Clara University has a “Young Scholars” program designed for high school students. Fees are comparable to the UC/CSU programs.