A four-year letter winner for coach John Nelson at St. Mary's College High School, Kevin was rated as the No. 384 high school player by Perfect Game USA before the 2013 season. He played in the 2012 Area Code Games for the Oakland Athletics squad.
Kevin was first-team all-state in 2012 and was a Contra Costa Times East Bay top 100 athlete in 2011-12. He was first-team Tri County Athletic League all-star in 2013, 2012 and 2011, the TCAL pitcher of the year as a sophomore and junior, and an honorable mention selection as a freshman.
As a junior, Kevin hit .495 with five doubles, one triple, six homers and 39 RBIs. As a pitcher, he was 10-0 with a 1.37 ERA in 14 games, with 114 strikeouts in 68.1 innings. He hit .307 (3-20) and was 3-4, 1.88 as a sophomore, and was 4-4, 2.33 as a freshman.
He also lettered four years in football and was a second-team all-league selection as a quarterback his junior season. An older brother, Matt Flemer, pitched at California and is now playing for Asheville of the Class A South Atlantic League in the Colorado Rockies organization.