cram school vs prep school

cram school

noun
  • A specialized school that trains students to meet particular goals, most commonly to pass the entrance examinations of high schools or universities. 

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 cram school and prep school occurred in a corpus of books? (source: Google Ngram Viewer )