Michael Schoch, Answers questions on Noodle
High Schools don't issue visas to students. You can learn about student visas from the U.S Bureau of Consular Affairs. Generally, you must first be accepted to a school. Once you're accepted the school will send you the application for a visa. A great explanation of the process is available from StudyUSA.
Your next step should be to contact Capistrano Valley High School at (949) 364-6100 to ask about its exchange programs and policies regarding international students. You can also learn more about the school on its website.
Best of luck and feel free to write back if you're still unsure!