For clarity: A guy starts speaking in a rude manner to an upperclassman (unknowingly). so the upperclassman's reply is:
いきなりタメ口って
一応 同中の先輩なんだけど
Translation:
Well, that's not very polite!
I'm actually your senpai.
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Sign up to join this communityThis って is a colloquial topic marker that is used in place of とは. とは/って used like this expresses the speaker's (negative) surprise, bafflement, etc. The senpai used って because he did not expect タメ口.