I guess the answer is: It depends.
I have once heard that McKinsey has this rule that the sum of Abitur, Vordiplom and Diplom (you're German, you understand) should not exceed 5. So in that case, you'd be screwed.
But I believe this is an urban myth. Maybe some of the McK alums here can confirm. I am sure that most firms will ignore bad highschool grades if everything else checks out nicely. At least all that I have worked for or with.
It would also be stupid not to do so. Because why would you not at least look at a very promising candidate just because some grade from half a decade ago were mediocre.
But you can expect some questions on that during your interview, particulalry if your grades were poor in Maths or Physics. I had a question during the final partner interview (the partner in question was ex-McKinsey): "Im Grundkurs in Mathe nur 12 Punkte? Was ist denn da schief gegangen?"
I guess the question was more intended to throw me off balance and see whether I shared his sense of humor, but as a kid in your early twenties, when your job is on the line, a question like that might suffice...