Allen attended Northern Guilford High School, where he played football, basketball, and ran track. In football, he was a standout on both sides of the ball for the Nighthawks football team. He was a 2009 high school All-American, selected by USA Today and Parade, and also played in U.S. Army All-American Bowl, returning one punt for 33 yards as well as one kickoff also for 33 yards.
In track & field, Allen was a state qualifier in the long jump (top-jump of 7.01 meters). As a senior, he competed in sprints. He took 24th in the 400-meter dash event at the 2010 PTFCA State Meet, with a time of 51.60 seconds. He also competed in the 100-meter dash, recording a personal-best time of 11.2 seconds. In addition, he was also timed at 4.56 seconds in the 40-yard dash.
Allen was regarded as a five-star recruit by Rivals.com and Scout.com, and was widely considered the top defensive back in the nation.