Go to TABS (The Association of Boarding Schools) website. There are several very academic schools in California (Cate and Stevenson). There is an application process. Those two schools are very selective and don't generally take kids mid-year. Here are others:
* Army and Navy Academy, Carlsbad, CA
* The Athenian School, Danville, CA
* Cate School, Carpinteria, CA
* Dunn School, Los Olivos, CA
* Flintridge Sacred Heart Academy, La Canada - Flintridge, CA
* Happy Valley School, Ojai, CA
* Idyllwild Arts Academy, Idyllwild, CA
* Midland School, Los Olivos, CA
* Monte Vista Christian School, Watsonville, CA
* Oak Grove School, Ojai, CA
* Ojai Valley School, Ojai, CA
* San Domenico School, San Anselmo, CA
* Santa Catalina School, Monterey, CA
* Southwestern Academy (CA), San Marino, CA
* Stevenson School, Pebble Beach, CA
* The Thacher School, Ojai, CA
* Villanova Preparatory School, Ojai, CA
* The Webb Schools, Claremont, CA
* Woodside Priory School, Portola Valley, CA