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, 2010
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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 19, 2009
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This describes someone who is constantly judging everyone and everything to be less cool than they are, because in their mind they're a true rebel in the know about everything. This kind of person mocks or rolls their eyes at anyone who they judge to be uncool, but only because they imagine that other "cool" people would hold such a view, and they feel they're in the know about how all the "cool" people think, and want to feel accepted by the cool crowd. They're simultaneously insecure and worried about maintaining their image at all times, and in case anyone might see them, they won't let themselves get really into or become enthusiastic about anything, lest that thing be thought of as uncool.

Also applies to hipsters on the excessively surly and douchey end of the hipster spectrum.
As an example of the too cool for school thought process:

**Sees someone being enthusiastically into science (or history, or literature, or math, or a movie, etc etc)....

Thinks subconsiously: "All the cool people think science is lame, and I want to be accepted by the cool people, so I should agree with them, so therefore, science must be lame, and I had better not have any interest in science."

Thinks consciously: "Pfff, what a dork!" Rolls eyes.
by phillybagel June 17, 2017
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