Why are there no maids at Devon when Finny returns?

There are no maids because of the war. We can infer that the "maids" are busy doing more important work, or that the school has simply decided to cut back in wartime.

“No maids,” I said. “After all, there’s a war on. It’s not much of a sacrifice, when you think of people starving and being bombed and all the other things.” My unselfishness was responding properly to the influences of 1942. In these past months Phineas and I had grown apart on this; I felt a certain disapproval of him for grumbling about a lost luxury, with a war on. “After all,” I repeated, “there is a war on.”