That's when Cooper's on-field persona emerges, the one that belies his infectious smile and mild-mannered nature.
Once Cooper buckles his chin strap, "He becomes a different person; that's when the dog comes out," said Prater, of Proviso West (Hillside, Ill.).
Cooper on the field is likened to a feisty watch dog: Once you enter his territory, look out.
"I don't know about that," said Cooper, a rising senior wide receiver and safety at Proviso East (Maywood, Ill.). "I am more vocal on the field; I'll get in a player's grill if I have to."
One thing is for sure: Both Cooper and Prater are hoping to make statements this week at the inaugural Champion Gridiron Kings, part of the ESPN RISE Games presented by Target, held at Disney's Wide World of Sports Complex in Lake Buena Vista, Fla.
The elite high school football event will feature 64 of the best senior prospects, mostly on the ESPNU 150 Watch List, in the game's seven skill positions -- quarterback, running back, tight end, wide receiver, linebacker, cornerback and safety.
The seven-on-seven competition Saturday will be the culmination of the three-day event, which will include performance training, coaching from several former NFL players, position specific training and skills challenges. ESPNU will televise the event at 7 p.m. ET Tuesday.
The Maywood, Ill., residents are used to each other. They work out daily at either one of their high schools or at nearby Melrose Park, and when they're not training, they hang at the mall and double-date. They struck up a friendship while attending fifth grade at Garfield Middle School and playing Pop Warner with the Maywood Bucks.
They went their separate ways in high school, despite living only two blocks apart.
"My mother wanted me to attend [Proviso] West; she liked the school," Prater said. "At first I was upset, but now I love it."
Prater, who will graduate in December, holds a solid 3.3 grade-point average (on a scale of 4.0) and has plenty of suitors. Recently, he whittled his list to 10 schools -- Alabama, Auburn, Illinois, Missouri, Miami, Michigan, Notre Dame, Oklahoma, Tennessee and USC -- but he has no favorites.
"I have to make a decision soon, at least before December," he said. "I'm trying to schedule visits now for the fall."
Cooper, an Illinois commit, is doing his best to recruit his friend to Champaign.
"[Recruiting] comes up occasionally; it's not like we talk about it all the time," Cooper said.
But lately, Prater is the talk of the town.
Earlier this month, the 6-foot-5, 205-pound wide receiver tried his hand at quarterbacking the West Panthers at the Downers Grove South seven-on-seven tournament. His presence under center could portend of things to come.
"I have a cannon [for an arm]," Prater said. "Wherever coach [Famous Hulbert] needs me, I'm there for the team. Coach wants to get the ball in my hands as much as possible."
And that's exactly what he did for the 2-7 Panthers last fall. Prater was the team's lone offensive threat, totaling 948 yards on 60 receptions (15.8-yard average) and nine touchdowns. He followed up his junior year by bagging MVP honors at a national combine in January in San Antonio.
Tom Luginbill, a former quarterback at Georgia Tech and the national recruiting coordinator for Scouts Inc., appreciates Prater's physical abilities.
"He's as big as a flamingo," he said. "Kyle has long arms and legs, and with that, a tremendous catch radius. He has a distinct advantage in traffic and the red zone but does not have exceptional speed."
If there's been one knock on Prater's game, it's been his foot speed.
"I've heard it all," Prater said. "So I did something about it."
Offseason workouts took on new meaning this year, emphasizing speed work. Tailored drills addressed his explosiveness. He increased his squat by 150 pounds from last year to 375 pounds. Recently, his hard work paid dividends when he shaved nearly two-tenths of a second off his 40-yard time (4.4).
"He's driven to be the best," Hulbert said. "He's self-motivated, and the knock was that he was slow. One time as a sophomore, he was caught from behind, and after that, he vowed not to let it happen again."
Personal achievements aside, Prater is poised to help Proviso West reach the postseason for the first time.
"We want to be the first team to do it; that's our goal," Prater said.
Cooper (6-2, 203) might be Illinois' most versatile player.
As a junior, the first-team all-state selection caught 40 balls for 600 yards and six touchdowns, and returned two kickoffs and two punts for scores as Proviso East reached the second round of the Class 7A state playoffs. Cooper, who likely will play safety in college, added 60 tackles, four interceptions and two sacks as an ace defender.
His Scouts Inc. report card is quite impressive:
"Cooper is a versatile skilled athlete that can play safety, wide receiver and stands out as a return specialist. He has great size and is physically impressive. Possesses good speed and has excellent range. He is a very solid football player that could line up either side of the football at the next level and should also excel on special teams."
Cooper's Proviso East Pirates return several key offensive players this coming season. "We'll make another run this year in the [state] playoffs; we have enough players to go deeper," he said.
When Cooper and Prater, both on the ESPNU 150 Watch List, return from the Gridiron Kings, the season will be less than a month away. Their high school teams will meet Sept. 12 in Maywood in a cross-town rivalry.
"It's always intense. I like that it's during warm weather; usually we play at the end of the season," Prater said.
Although the suburban Chicago duo likely won't play on the same squad in Florida this week, both are up for personal challenges.
"If these are the best players, I want to see how I measure up," Prater said of the Gridiron Kings' stacked talent pool.
Added Cooper: "It'll be great experience. I want to see the top skill position players and where I stand."
Christopher Lawlor has covered high school sports for more than 20 years, most recently with USA Today, where he was the head preps writer responsible for national high school rankings in football, baseball, and boys' and girls' basketball. He also worked for Scholastic Coach magazine, for which he ran the Gatorade national Player of the Year program for nine years. Lawlor, a New Jersey resident, grew up in Rochester, N.Y., and is a graduate of St. Bonaventure University.