There are many idioms throughout the book Wonder by R.J. Palacio but I am going to focus on one specifically. This idiom I found deals with the main character August "Auggie" Pullman. The term "like a lamb to slaughter" is used referring to Auggie's experience of going to main-stream school for the first time. This term normally refers to someone who is ignorant of the danger that awaits them.
Auggie is born different and up until the time of the book (Auggie is going into the 5th grade) he had never been to a traditional school. Auggie has a facial deformity which has caused him to undergo many surgeries so he never had the ability to go to school before. Now that he can he is completely terrified of how he will be perceived and treated by his peers. Despite his fears Auggie still did not understand how difficult the beginning of his time at Beecher prep would be until he got there. Hence the idiom "like a lamb to slaughter".