What kind of formula does Medza create to encounter anyone performed in the exams?

Madza stated the fact, and left his response unopen to further conversation. He jsu shut his questioner down.

From the text:

It was in July, if you remember, and I (together with several others of my class, whose names I now forget) had just failed the oral in my baccalaureat exams. I am sure you have not forgotten the formula I discovered after this set-back to counter anyone who asked me how I had done : it always amused you. Oh, I was ploughed — in the best tradition,’ I would say. The words in the best tradition always came out in a faintly assertive tone : they served as a kind of alibi, and relieved both my questioner and myself of the need to go through all the misfortunes that had led up to this particular failure. (I have a nasty feeling that, as a result, I have become a kind of professional failure; but that is another story.)