Prior to Season 8 coming along and introducing the School of Friendship, were there any episodes before the season where a character states that Friendship isn’t something that could ever be taught in schools?

If so, then which episode? Which character? And what was the exact quote?

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Not sure about an exact quote (though I seem to remember there being one), but Celestia tells Twilight in the very first episode to get out of the library and make some friends. Not quite the same thing, but the implication is that you can't learn friendship in an academic setting.

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Which is, of course, bollocks. Minuette, Twinkleshine, and Lemon Hearts all learned friendship well enough in an academic setting. The difference between them on the one hand and Twilight and Moondancer on the other was that they made more time for socializing outside of their coursework (which did not in itself focus on teaching social skills), and that worked for them. But explicitly setting out to teach social skills is not a bad idea, progressive education (Montessori et al) leans heavily into that. It is implied that Twilight sought initially to apply prog-ed principles in her school's structure and curriculum but set that aside to conform to EEA guidelines--then returned to the initial principles when she threw them out. And at the end of Friendship Is Magic, Part 1, Celestia frames Twilight's new lot in life as a course of independent academic study. In part that is humoring Twilight's preconceptions, but in part it is also true.