noun (n)

• any day on which school is in session(noun.time)

  Example:
  Go to bed early because tomorrow is a school day.
  

  source: wordnet30
• the period of instruction in a school; the time period when school is in session(noun.time)

  Example:
  When the school day was done we would walk home together.
  

  Synonym:
  school, schooltime
  

  source: wordnet30