Question: I don't study in the United States right now, and I want to continue my high school education there. I am finding a boarding school in California, but since they are private, will their standards be lower and will it affect my college decision? Also any suggestions of good boarding schools?
Private schools vary in quality just like public schools. Some public schools will offer you a premium education that can prepare you for the Ivy League and other so-called "elite" colleges. Likewise, some privates will vastly outperform many publics. You'll have to investigate on a school-by-school basis.
As for candidates in California, check this site:
You can also use your favorite search engine (try Google.com) to find out a lot more about private schools in California. Just enter "California private schools" or something similar, maybe adding a particular county for detail.