In A Separate Peace, what happens to Devon in chapter 13?

I believe this question is asking to describe what is physically different about Devon School in chapter 13.
The answer to this question can be found in the early parts of the chapter. The reader knows from earlier in the book that the book is set during World War II—specifically, the 1942-1943 school year. These dates put the war effort in full stride. The United States is training massive numbers of troops and support units and needs space to do so.  
The narrator tells readers in chapter 13 that the Far Common is donated to the military for the war effort. The area is now full of troops and military equipment for training purposes. The specific use of the area is to teach parachute riggers their trade.