middle school vs schooltime

middle school

noun
  • An educational level or stage between primary school and secondary school. 

  • A school which crosses the traditional divide between primary school and secondary school. 

schooltime

noun
  • Time spent in school; classtime 

  • Time spent at school; schooldays 

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