During his two seasons in Stanstead, Jankowski quickly gained height, growing to 6'2" tall, while it was his play that caught the attention of scouts. He was eventually selected in the 2012 OHL draft, taken as the 11th pick in the 7th round by the Saginaw Spirit though he declined to join the team. The National Hockey League (NHL)'s Central Scouting Bureau ranked Jankowski as the 43rd best prospect for the 2012 NHL Entry Draft in its final ranking, an improvement from his 74th place rank in previous reports. Other analysts ranked him higher, including Craig Button of The Sports Network, who placed him 41st. The Calgary Flames selected him in the first round, 21st overall. He is the highest-ever draft pick out of a Canadian high school. The Flames received some criticism for going "off the board" and selecting Jankowski well above his expected draft place, but assistant general manager John Weisbrod stated that while Jankowski's talent was "raw", other teams were certain to select him in the first round if the Flames did not. General manager Jay Feaster argued that Jankowski was one of the most talented players available in the draft. At draft time his stats were 6'2.5", 168 pounds.