HIGH SCHOOL ATHLETE OF THE WEEK : Sparks Flies in Role as Substitute Tailback : Football: Despite sitting out most of the second half, the Montclair Prep junior scored six touchdowns.

In either the game’s latter stages or its aftermath, something occurred to Derek Sparks.

It might have been as he stood on the sidelines Friday night for most of the second half of his team’s 55-14 walloping of St. Genevieve.

It might have been when he finally sat down and stared at the telling numbers on the statistics sheet.

Or it might have been Monday morning in class at Montclair Prep:

Man, he thought to himself, I could’ve gained 500 yards.

I could’ve scored 15 touchdowns.

To be sure, there is an element of hyperbole in those thoughts. There is, however, an element of reality too.

He might have been able to do all that.

As it was, Sparks, a junior fullback substituting for injured tailback Michael Jones, carried the ball just 11 times Friday night.

For 215 yards.

And six touchdowns--five on consecutive carries.

Sparks’ scoring jaunts covered 38, six, one, 17, 70 and 45 yards.

Having Sparks fill in for Jones, who was resting a bruised knee, is the high school football equivalent of having Will Clark pinch-hit for Kevin Mitchell: There’s not a large decline in ability.

Sparks, normally the fullback, came into the game with 664 yards in 74 carries--along with 11 touchdowns--to lead Valley rushers in yardage. Jones had 656 yards in 85 carries.

Still, the Sparks explosion surpassed the expectations of even the most optimistic Mountie supporters. He averaged 19.5 yards a carry and scored a touchdown every 1.83 times he touched the ball. Sparks also increased his Valley lead to 879 yards in 85 carries with a Valley-best 17 touchdowns.

“As you know, Michael was out,” Sparks said, “so I had to take on the load. I had a nice little talk with the line before the game and said, ‘Hey, you know, Mike’s out, so let’s pick it up. If you guys have a great game, I’ll have a great game.’ ”

If, in the tradition of O. J. Simpson, a running back should compensate his line financially for yardage gained, one could expect to see the Montclair Prep linemen strutting around school wearing fur coats and bejeweled rings and being chauffeured to and fro in limos.

Such riches will have to wait, though. “If I had that type of money,” he said, laughing. “In the meantime, I’ll have to use my lips and thank them.”

Thankful for the arrival of Sparks at Montclair Prep this season is his coach, George Giannini, who is beginning to accept such performances with as much nonamazement as he would a sunrise.

“He really has a great ability to run with his shoulders to the end zone,” Giannini said. “Even when he makes a cut, he’s pointed toward the end zone. That makes him a great runner.”

Almost too talented for the competition, it’s becoming evident. Sparks’ efforts landed him an early breather; he sat out almost the entire final two quarters when the Mounties ran only seven offensive plays. The only running Sparks did was between the water supply and the sideline, from where he watched the game.

Which is why his thoughts turned to what might have been.

“We could have scored 100 points,” he said. “We can’t let those things pass. It got really fun out there.”

Discretion prevailed and Montclair Prep shied away from letting Sparks humiliate the Valiants further. The Mounties scored just twice in the second half.

So how does a junior in high school celebrate such an evening? Is it chili fries for all, on him? Does he go out and cut a rug at a local dance club?

Nothing so eventful. Just a quiet night at home.

Sparks and his cousin, Leland, the team’s quarterback, relaxed with offensive tackle Donovan Roy and watched a videotape--”Eddie Murphy’s Greatest Hits From Saturday Night Live.”

His particular favorite is the skit involving Murphy as James Brown in a whimsical look at “James Brown’s Celebrity Hot Tub.”

But on this Friday night, in the Valley, at least, Sparks himself was the celebrity. And for the next five weeks, the joke is on opposing defenses.