Wordnet 3.0
NOUN (2)
1. 
 any day on which school is in session; 
- Example: "go to bed early because tomorrow is a school day"2. 
 the period of instruction in a school; 
 the time period when school is in session; 
- Example: "stay after school"- Example: "he didn't miss a single day of school"- Example: "when the school day was done we would walk home together"[syn: school, schooltime, school day]
WordNet (r) 3.0 (2006):
school day
    n 1: any day on which school is in session; "go to bed early
         because tomorrow is a school day"
    2: the period of instruction in a school; the time period when
       school is in session; "stay after school"; "he didn't miss a
       single day of school"; "when the school day was done we would
       walk home together" [syn: school, schooltime, school
       day]