Walker was a three-year starter at North College Hill High School, where he was a consensus Top 10 high school recruit and a teammate of O.J. Mayo. As a sophomore and junior in 2005 and 2006, Walker helped North College Hill to back-to-back Ohio Division III state titles and was named the Most Valuable Player of the 2006 State Tournament. In 2006, North College Hill finished third in the USA Today Super 25 High School rankings,[8] with Walker averaging 21.7 points and 10.1 rebounds per game. He was selected the USA Today National Player of the Week on March 27, 2006. In the fall of 2006, Walker graduated early after it was ruled that he had used all of his high school athletics eligibility the previous year.

Walker also helped the USA White team to a 5–0 record at the 2005 USA Youth Development Festival in San Diego, California, where he played alongside Ohio State freshman Greg Oden and Kansas freshman Darrell Arthur. He was the only three-time Most Valuable Player at the Reebok ABCD Camp in Teaneck, New Jersey.