This question is a good question. Evidence that hinted toward McCandless's eventual solo journeys and "later behavior" are plentiful in the text; however, most of the really solid examples are from McCandless's college years. The question makes things harder by specifying high school or earlier; however, there is evidence that foreshadows his future. Chapter 11 has a really great paragraph about an experience that McCandless had in third grade. He scored really well on a standardized test, and he was then put in an accelerated program. McCandless wasn't happy about it and tried everything he could to get out of the extra work. His teacher saw what was happening and informed McCandless's parents that "Chris marches to a different drummer."
In the third grade, after receiving a high score on a standardized achievement test, Chris was placed in an accelerated program for gifted students. “He wasn’t happy about it,” Billie remembers, “because it meant he had to do extra schoolwork. So he spent a week trying to get himself out of the program. This little boy attempted to convince the teacher, the principal—anybody who would listen—that the test results were in error, that he really didn’t belong there. We learned about it at the first PTA meeting. His teacher pulled us aside and told us that ‘Chris marches to a different drummer.’ She just shook her head.”
The third-grade experience shows that as an 8 or 9 year old, McCandless already had a stubborn and independent spirit.
McCandless's sister, Carine, gives Krakauer a good testimony about McCandless in chapter 11 as well.
“Even when we were little,” says Carine, who was born three years after Chris, “he was very to himself. He wasn’t antisocial—he always had friends, and everybody liked him—but he could go off and entertain himself for hours. He didn’t seem to need toys or friends. He could be alone without being lonely.”
Her comment shows readers that McCandless had, from an early age, the inclination and ability to spend long periods of time by himself. He was perfectly capable of being social; however, McCandless preferred solitude.
She also states that McCandless showed a "wanderlust" at an early age too. He loved simple camping, and the lengthy camping trips were always his favorite.
“We camped out of the back of the truck, the Chevy Suburban,” Walt explains. “Later we bought an Airstream trailer and traveled with that. Chris loved those trips, the longer the better."