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.
It was my English teacher and The Jungle too. My class would always end up in debates so I think he could tell I wanted something political to give me that push into leftism. He was a pretty great guy, I should really see how he's doing these days