JaShaun Smith hit a 3 to reduce Longwood's deficit to 25-15 with 6:49 before halftime, but Cortez Mitchell made a layup and Fleming buried another 3 for a 15-point lead. Fleming's last 3 of the half put the Buccaneers up 42-23 at the break.
Charleston Southern extended its lead to 61-30 with a 19-7 run after intermission with Keeling scoring eight and Cortez Mitchell adding seven. Mitchell finished with 13 points and McConico scored 10.
Smith led Longwood (3-11) with 15 points.
Mannion, who recently reclassified and is ESPN's No.11 overall player in the class of 2019, chose Arizona over Marquette.
He'll fit in with Sean Miller's plans for a faster tempo on offense.
The cream of the crop of American high school basketball will congregate in Colorado Springs from Oct. 5-7, as USA Basketball unveils its newly expanded Junior National Team minicamp in collaboration with the NBA, NCAA and NBPA.