For me, it was mainly the philosophy and politics teachers. Boy oh fucking boy, for some reason, every single one of my philosopy and PolSci teachers were... interesting personalities to say the least. It was our philosophy teacher who introduced us to the Labor Theory of Value, oddly enough, and the issue of economic inequality. And as for the PolSci-Side, two teachers stand out: One grew up in the GDR and was very, very nostalgic about it (with some fair criticisms, especially concerning the Stasi), and the second, and final one in my case, was an irony-poisoned super-leftist with a gigantic disdain for the rich, the state and capitalism.
My education was Catholic - so slowly realizing that I had been completely indoctrinated into a comically evil set of beliefs primed me to later understand that the same was happening on a societal level with respect to capitalism.