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2.
Left over from when you'd stay up too late when you had school the next day, but applied to adult life; For example going clubbing when you have work the next day
Andy: Ah man I had a massive hangover this morning, was nearly late for work!
James: Clubbing on a schoolnight? Tut tut!
by Calamity James August 21, 2009
 
1.
To have commitments the next day, esp. in the morning, eg interviews, work, dates etc, that prevents you from doing something (usually fun and stupid) on that night.

Originated, (obviously) when having to go to bed early because school was the next day.
I'd love to get drunk and stay out all night with you guys, but it's a school night!
by ph@11u55y June 26, 2009
 
3.
An excuse to prevent from doing an immediate action or otherwise on a later date.
See, I would punch you in the face for steeling my tater tots, but you know what .... its a school night
by Larbac August 16, 2010