As a wide receiver, he has made an immediate impact in his true freshman season. He was selected All-American as a return specialist by CBS Sports in 2015. The 2015 Gators got off to an unexpected 6–0 start after entering their first four games unranked in the AP Poll. Their fourth victory was a thrilling 28–27 come-from-behind victory over Tennessee as Will Grier completed a pass on 4th-and-14 to Callaway for a 63-yard touchdown, and Tennessee could not respond as they missed a 55-yard field goal attempt as time expired. Callaway finished the 2015 season as Florida's leading receiver with 35 catches for 678 yards and four touchdowns.
In January 2016, Callaway and quarterback Treon Harris were suspended from the team. Florida head coach Jim McElwain said the suspensions were related to "schoolwork." It was later revealed that the suspensions were related to an accusation of sexual assault. A woman accused Callaway of sexual assault and Harris of attempted sexual assault in December 2015. The woman reported the alleged incident to Florida's student conduct and conflict resolution office, but did not report the incident to either Gainesville police or University of Florida police. The University of Florida suspended both players for violating the school's code of conduct policy. Both players remained suspended until June, when they returned to campus to take classes and work out at the school's facilities. Callaway was found not responsible of sexually assaulting the female student by an independent hearing officer in a Title IX hearing. The ruling was made by attorney Jake Schickel, a former track and field athlete at Florida who has donated money to Florida's football and basketball programs. Schickel cleared Callaway of three violations of Florida's student conduct code, which included endangering another's health or safety, sexual assault and sexual misconduct, and conduct causing physical injury.