As a senior, Costello led Elkhorn South to the 2017 Nebraska Class B State Tournament, averaging 18.6 points, 3.9 rebounds and 3.5 assists per game to earn first-team Class B honors from both the Omaha World-Herald and Lincoln Journal Star. Costello shot 43 percent from the field, including 67 3-pointers, and 78 percent from the line for the Storm. He had a pair of 30-point games this winter in helping Elkhorn South finish with an 18-7 record, including a 39-point outburst against Lincoln Southwest. In that game, Costello went 9-of-11 from 3-point range, one of seven games where he had at least four 3-pointers in a game in 2016-17.
A three-time all-state pick, he earned second-team All-Class B honors from the Omaha World-Herald and Lincoln Journal Star as a junior, averaging 18.0 points and 3.1 rebounds per game and was an honorable-mention pick as a sophomore, as Elkhorn South went 21-5 and won its first Class B state title.
