At least a few of the teens are 18 years old and will be charged as adults, Mason said.
"This is very serious," the deputy said. "It went too far, and we're serious about taking care of it."
A dead ewe was found Wednesday morning dangling from the goalposts on the South Sevier County High School football field. Someone apparently shot the animal in the head.
The South Sevier and Richfield high school football teams played later that night, and Mason said the sheep killing was intended as a prank on South Sevier, whose mascot is a ram. South Sevier won 26-6.
The sheep came from a farm north of Richfield, Mason said, and was killed in a field there. The teenagers did not have permission to be on the farm or to take the sheep.
The sheep was fastened to the goalposts by chains with two padlocks on them. Investigators traced the locks to a local Wal-Mart and found security camera footage of two boys buying the locks. Mason said the sheriff's office also retrieved a partial license plate number from the footage and found those same numbers on a vehicle in the parking lot at Richfield High School.
Mason said some of the seven suspects participated in the sheep theft and killing while others were present only for the delivery to South Sevier.
"That's what we're trying to determine who did what so we can determine what charges would be appropriate," Mason said.
The sheriff's office had not arrested anyone in the case as of Friday morning.