A Separate Peace has a dark tone throughout the novel because of the back and forth feelings that Gene has about Finny. It's a good vs. evil theme which transpires through the carefree innocence that occurs in the summer time and their feelings of freedom--and then takes the reader through the loss of innocence when not only does Finny get hurt, he eventually dies because of surgery complications.
The tone almost gives off a sense of foreboding (fear of what's to come). The reader feels for Gene, and is struggling right along with him and his jealousy in the beginning and his guilt throughout the rest of the story. This is not a happy, light story. It's intense.
Is there a quote you can find in the novel that indicates tone?