The obvious solution is to pay everyone when they reach the achievement levels, not just the people who fell behind and then caught up.

Having worked with kids before: Kids and young people will game the hell out of any incentive structure put in front of them with terrifying efficiency. If anyone gets the idea that they can make some money by falling behind and then catching up later, a significant number of students will start falling behind so then can execute whatever catch-up maneuvers get them their payout.

So sidestep the issue completely and pay everyone when they reach the achievement threshold.