Should I take the TOEFL exam if I have graduated from a high school in the U.S.?
Every school words the requirements, if any, for the TOEFL a little differently. For example, Harvard GSAS:


  Applicants whose native language is other than English and who do not hold the equivalent of a US Bachelor's degree from an institution at which English is the language of instruction must submit scores from the Internet based test (IBT) of the Test of English as a Foreign Language (TOEFL)


A quick look suggests that in general native language and the language of your Bachelor's degree is what matters and not the language of your high school.