Almost every art school across the country offers a pre-college program, which is a summer intensive course for high school students, usually 3-4 weeks long, through which you will earn college credit and get a huge boost in your art skills. I highly recommend you consider these options if you are serious about your artmaking practice, especially if you plan to take AP Studio Art and/or apply to art school.
FREE Academy of Art Pre-College Art Experience in San Francisco (BART-able from Castro Valley). Note: I do NOT recommend Academy of Art for college, because I have known countless students who've felt that the quality of instruction does not match the high price of tuition, but free is free, and several students have positively reviewed their pre-college classes.
California State Summer School for the Arts (CSSSA) at CalArts in Valencia (Southern California)
CalArts Extended Studies (in Valencia) -- specializing in animation, character design, and storytelling
ArtCenter Extension summer courses for teens (in Pasadena)