high school vs prep school

high school

noun
  • An institution which provides all or part of secondary education. 

  • Secondary school. 

prep school

noun
  • A private or public school intended to prepare its students to get into prestigious universities. 

  • A private primary school which prepares its pupils for the common entrance examination most commonly at the age of thirteen, and subsequent entry into public school. 

How often have the words high school and prep school occurred in a corpus of books? (source: Google Ngram Viewer )