Dennis attended Saint James High School in Saint James, Louisiana where he was a student and a standout in football. He was a first team All-District 9-3A selection, an All-Bayou selection, and an All-Metro selection, and the Louisiana Football Magazine named him as a first team All-State selection. He was also a teammate of New York Giants cornerback Corey Webster who played quarterback at the time.
In 1999 as a junior Dennis was Webster's go-to receiver and managed to catch 61 passes for 1,031 yards and 14 touchdowns. Against E.D. White he caught 7 passes for 265 yards and 3 touchdowns, which earned him the WAFB High School Football Player of the Week award. He was also recognized by USA Today for having one of the best performances in Louisiana that week.
As a senior, he caught 45 passes for 970 yards and 13 touchdowns. In the season opener against rival school West St. John, Dennis had 6 catches for 47 yards and 3 touchdowns, returned an interception 60 yards for a score as time expired and also returned the opening kick-off 100 yards for the score which was unfortunately negated by a penalty. For his efforts, he was named River Parishes' Prep Player of the Week. He graduated from Saint James High School in 2001.
After being heavily recruited by universities such as San Diego State, Colorado and Mississippi State, Dennis eventually committed to Kansas State University in February of 2001.