Born in West Little River, Florida, Cook attended Miami Central High School, the same as former Seminole and current Atlanta Falcons Pro Bowl running back Devonta Freeman. Cook did not join the football program until his sophomore year, figuring he had no chance to supplant senior running back Devonta Freeman, who led Central to a Class 6A state title in 2010. Cook played as a running back and defensive back under coach Telly Lockette. In his junior year, he shared carries with Joseph Yearby, who is his best friend. On running downs, Yearby played wildcat quarterback and Cook lined up at running back. As a senior in 2013, Cook rushed for a county-best 1,940 yards and 34 touchdowns on 177 carries (11. 0 yards per rush) and intercepted three passes on defense. In Central's Class 6A regional final win against Palm Bay Heritage, Cook ran for 244 yards and three touchdowns as Yearby fractured his fibula in the first quarter of the game. From that point on, Cook led his team to the Class 6A state championship by rushing for 223 yards and four touchdowns in a 52–7 win over Seffner Armwood, helping the Rockets win their third state championship in the past four seasons and become the first Dade team to advance to four consecutive state finals. For his season efforts, Cook was named Mr. Florida Football by the Florida Dairy Farmers Association, becoming the third recipient of the award since its inception (back in 1992) from Miami-Dade County, joining former Miami Northwestern quarterback Jacory Harris and former Miami Norland running back Duke Johnson. Cook finished his prep career with 4,267 rushing yards and 64 touchdowns, while leading Miami Central to a 52–5 record. His senior season accolades included USA Today All-USA Football Team, 247Sports Second Team All-American and All-State Class 6A first-team. Following his senior season, Cook was invited to play at the 2014 Under Armour All-America Game, where he rushed for 78 yards and a touchdown on just eight carries for Team Nitro. Cook also participated in "The Opening", an all-star summer prospect camp held on the Nike Campus.