What would be some good short stories or poems to teach alongside A Separate Peace by John Knowles?
Perhaps something that involves similar themes as the novel, such as themes of complicated friendships, jealousy, fear, loss of innocence, or identity.
The themes you have drawn out of this novel seem very accurate, central to the meaning of Knowles' novel and broad enough also to connect this book to other works.
Looking for short pieces to teach along side this book, Ernest Hemingway's stories come to mind. "In Another Country" is a story of complex friendships and loss of innocence which offers a similarly subtle exploration of these themes. The story also deals with the idea of personal loss.
After all, the story suggests, attempts to avoid loss are only temporary.
There are connections to be made between Knowles work and two famous American poems, "O Captain! My Captain!" and "The Road Not Taken". The exact connections made will depend on the reader, but these works deal with ideas of individuality and leadership in ways that relate to Finny's relationship with Gene and to Gene's final realizations in the novel.
Whitman's "Song of Myself" also offers a number of moments where the narrator identifies with others while also distinguishing himself. This act of identity formation is very much in line with Gene's conflicts.
Although they concern World War I and not World War II (as A Separate Peace does), I would consider looking at the poems of Wilfred Owen and Seigfreid Sassoon. As the Knowles work does, they explore the line between childhood and manhood against the backdrop of war. The short novel The Prime of Miss Jean Brodie by Muriel Spark offers a girls' boarding school contrast to Knowles's story. It also examines the effects of war on coming-of-age, but from a female perspective.