What are some lessons that students should learn throughout the novel The Boy in the Striped Pajamas?

After reading The Boy in the Striped Pajamas, students should have learned about human nature, genocide, and innocence.
Human nature is a major theme in the novel. Students can contrast the kindness of Bruno and Shmuel with the horrors the Nazis are perpetrating on the people in the concentration camp. Bruno and Shmuel have many things in common—including a birthday—and are both kind and caring boys who have been isolated from their former lives. The author contrasts a German boy with a Jewish one to show that people don't often start out bad. Rather, it's choices and circumstances that give way to problems like judgement, prejudice, and hate.
Innocence is another major point that students should understand after reading the novel. Bruno can be seen as innocent of his father's crimes, and the Jewish people locked up in Auschwitz are innocent of any crime. The two young boys are also too innocent to understand what's going on. They don't see Auschwitz as a death camp; when Shmuel can't find his father, Bruno disguises himself as a prisoner and sneaks in to help him find his father. This, of course, leads to Bruno's death along with Shmuel.
Genocide is an important theme in the novel. While Bruno sees the death camp as a farm, readers will understand its true purpose. When Shmuel's father goes missing and when Bruno sneaks in and the two boys are put in a gas chamber, students can see some of the horrors of genocide. It adds a personal touch to well-known history that may not resonate personally if it's only learned from a textbook.

Boyne's novel about a nine-year-old German boy who develops an unlikely friendship with a Jewish boy who is confined behind a barbed wire fence at Auschwitz has several significant lessons. This story is a cautionary tale of how radical government policies can propagate prejudice that results in the inhumane treatment of a specific group of people. The Nazi's Final Solution persecuted people of the Jewish faith and resulted in the deaths of approximately six million Jews.
Another lesson expressed throughout the novel is the idea that people should treat others the way they want to be treated. Unlike the other characters in the novel, Bruno's innocence allows him to treat everyone with respect and compassion.
Another lesson deals with the power of love and friendship. Boyne illustrates how love and friendship can overcome the most inhumane, dangerous environments. Despite the hate and brutality regularly surrounding them, Shmuel and Bruno develop a close friendship that withstands the test of time. Even though they both die at the end of the story, their bond transcends the physical realm.