You failed 9 subjects before you graduated from college.
I read up that its necessary for this to become "you had failed" since there are 2 past tense verbs, and because the "had" gives context that failing the 9 subjects happened before graduating.
But!! I can already completely understand the sentence without the necessary "had". I don't see any other interpretation other than "failed 9 subjects" happens before "you graduated", as that is a logical flow of events, and because there is already a "before".