But the fact that he is holding back (so he won't be found out and , using just enough of his power and strength to not only be competent, but to actually win the state championship is a calculating use of his powers for maximum personal gain, at the expense of others (who might have otherwise won, who might otherwise get a scholarship, had the chance to play quarterback, etc.) He should know it, his parents should know it, and even if Clark insists on doing the wrong thing for an ego burst, his parents should have put an end to it. Even if they, as Martha said, wanted to feel like he was normal for a little while, their son shattered an opposing player's collarbone. They almost snapped out of it, but he gave them a little song and dance - and they let him (no - they gave him their blessing) to go back out on the field even though he could just as well have injured more people, as at that point, Clark had no proof that their plan to neutralize Mxy was going to work.
At least football season is over now.