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1.
A state in which a person thinks him or herself superior to everyone else in a given group or in general. Generally used sarcastically.
Ben Stiller: "You may think you're too cool for school. But I got a news flash for you...you AREN'T."
(Zoolander)
by Nick D November 03, 2003
 
2.
A phrase originating in the 1960's Counterculture.

It describes a person who has "tuned in, turned on and dropped out." In other words, it describes a person who has submerged themselves so much in the Counterculture (fully embracing a wide range of drugs- particularly psychedelic drugs, communal lifestyles, anti-materialism, radical politics, anti-authority and particularly anti-Police attitudes, alternate codes of dress and personal hygiene and often anti-social behaviors), and is thus "so cool," that they are largely unfit for and unable to participate in conventional society.

The phrase was invariably complimentary.

More recent usages tend towards extreme forms of irony. Often applied with considerable sarcasm as a form of insult it generally means a person who is deemed to have an inflated self-image. If the person displays any actual Countercultural leanings, they tend to be of the most mild possible variety.
"Dude, the Hells Angels are totally righteous. They're so out there, man, they're, like, too cool for school."

"That kid didn't come to my party. What does he think he is, too cool for school?"
by The Egoist December 30, 2009
 
3.
1. Overconfident, cocky, sadiddy or conceited.

2. Believing that you know everything to the point to where it can harm you or work against you.

3. To be too smart for your own good or too smart by half, meant either literally or ironically.

4. To be in a state in which you think you’re better than anyone else in general.

NOTE: AS AN IDIOM, IT IS SARCASTIC.
Sadiddy people are usually too cool for school.
by Mr. Terrence L. Trezvant July 10, 2008
 
4.
Term used for a person who thinks they are somewhat superior to other people.
Tim R is too cool for school, look at his Warrick Cappa shoes!
by Ben April 05, 2005
 
5.
A student who is so cool that he/she is beyond the competition of being cool. It was mentioned once in a very popular sitcom.
Sarah is always absent. She is too cool for school.
by purplepizza January 30, 2014
 
6.
What you feel about you and how others may perceive you. If you have faith in yourself and others have it in you, you sre too cool for school!
Your freind hit how you are feeling on the button. They are now too cool for school because they know more than anyone else!
by The entirety of my heart June 18, 2009
 
7.
Also can mean someone who deliberately disobeys rules in a bid to look cool. Mainly used at school i think.
Boy1: Have you done any revision this holiday?
Boy2: No i just went out every nite
Boy1: Oooo...too cool for school
by flashermonkey April 13, 2004