Good GPA? Wear what you like, except distractions (naked) mean back-of-class seating. Bad GPA? You wear a one-size-fits-all jumpsuit made of faded rose-colored velvet.
Good GPA? Eat what you like. Bad GPA? You get that squishy nutriloaf thing normally reserved for misbehaving prisoners.
Good GPA? You get first choice for slots in sports teams, drama club, choir, cheerleading, and all the rest. It doesn't matter if you suck at the activity. Bad GPA? No participation permitted.
Enforce prom date pairs having a minimum average GPA. Those with the highest GPA can pick anybody. Those at the midpoint can pick the better half of the school. Those at the bottom get only one choice, and will thus need to make nice with the top-performing student.
Good GPA? Wander in and out of school as you please. Bad GPA? You pass through metal detectors, get a pat-down, and are sniffed at by the drug-detecting dog.
Good GPA? Move around and chat with friends during lunch. Bad GPA? You get assigned seating and must eat in total silence.
People will invent new ways to attempt dominance. Brutally crush this for any student who fails to perform academically.
Agreements can be made to extend incentives beyond the school:
Good GPA? You can drive a car at age 14, with reduced-rate insurance and school-provided training. Bad GPA? You must wait until age 25.
Good GPA? The local cops welcome you to go shooting with them. You get any needed state licensing at age 12. Bad GPA? Forget it; no 2nd amendment for you!
Good GPA? The local mall welcomes you by yourself. Bad GPA? You are not permitted entry. (Moderate GPA: only with parents)
Good GPA? Public transportation is free. Bad GPA? You don't ride without a parent. (Moderate GPA: standard fare applies)
Good GPA? Wander the city as you please. Bad GPA? There is a curfew.