a day of the week when students go to school
Example:
- I get up very early on school days.
the part of the day that students spend at school
Example:
- The school day ends at 3 p.m.
- Her school day started with a spelling test.
the period in your life when you go to school
Example:
- She hadn't seen Laura since her schooldays.
- Her schooldays had definitely not been the happiest days of her life.