2009-10: Slater Koekkoek skated for the Notre Dame Hounds Midget AAA team in Saskatchewan. He scored 16 goals with 27 assists and 91 penalty minutes in 44 games. In the SMHL playoffs he scored 3 goals with 4 assists and 6 penalty minutes in 13 games. Koekkoek was selected by Peterborough in the first round (7th overall) of the 2010 OHL Priority Draft.
2010-11: Koekkoek played in 65 games as an OHL rookie with the Peterborough Petes, represented Canada at the 2011 U18 World Junior Championship, and skated for the gold medal-winning Team Ontario squad at the 2011 U17 World Hockey Challenge. On a Peterborough team that won just 20 games, finishing last in the East Division, Koekkoek scored 7 goals with 16 assists and was minus-29 with 67 penalty minutes.In seven games with fourth-place Canada at the U18 WJC, he scored 1 goal with 1 assist and was minus-five with 2 penalty minutes. Koekkoek scored 1 goal with 2 assists and 8 penalty minutes in five games at the U17 WHC.
2011-12: Koekkoek skated in 26 games for the Petes – suffering a season-ending shoulder injury at the end of November. He scored 5 goals with 13 assists and was -7 with 17 penalty minutes. Without Koekkoek, Peterborough finished fourth in the East Division but did not make the OHL playoffs. Koekkoek was ranked 23rd amongst North American skaters in Central Scouting's final rankings prior to the 2012 NHL Draft.
Koekkoek is primarily an offensively oriented defenseman possessing a heavy and accurate shot. He also has solid playmaking ability, with the ability to make a good first pass out of the defensive zone as well as accurate passing in open ice. At this point, Koekkoek projects as a second-pairing defenseman with some potential to be a power-play quarterback.
Koekkoek will likely spend another season in the OHL before turning pro in 2014-15.