prep school vs 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. 

secondary school

noun
  • School with education level between primary school and university. 

  • In the UK and Ireland, a state school attended between the ages of 11 and 16 or 18. 

  • The federally designated, graduation-separated classification of grades 9–12 (approximately ages 14–18), regardless of whether they are compiled together in one school or separate from the other grades. 

  • Middle school; junior high school; a school that one attends between the ages of 12-17 (equivalent of grades 7-10 in North America, and years 7-10 in Australia). 

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