To obtain your high school transcript from Oakland's Grant High School in California, you should contact the school's registrar's office directly. Their contact information (phone number and email address) should be available on the Oakland Unified School District (OUSD) website. Searching "Oakland Unified School District Grant High School" on the internet should lead you to the correct page.
You may need to fill out a request form and potentially pay a fee. Be prepared to provide identification and potentially information verifying your identity.