Originally in French as OSS 117 à L'éCole, #76.
Following a clever simulation, a network of Russian spies operating in New York makes an American expert in slang believe that he killed an FBI man. He is offered sanctuary in the USSR, the real purpose being to have him teach American slang in a spy school located in Ukraine. OSS 117, having learned of this affair, takes the identity of this expert and manages to pass himself off as him with the objective of seizing the file of spies trained in this school..."