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.
The only instance of a teacher getting me to think about leftist stuff at all was my political studies teacher stalwartly defending the position that CEOs deserve to make 3.000 times the money of a regular employee because they just work that hard.
And I really cannot tell you if she meant it or if she was laying it on extra thick so we would actually think about the topic.