Stands for "based on previous wrong answer". BOPWA can sometimes save a student's grade on a rather complex math problem if they screw up on an early step in the problem. When BOPWA is written next to a problem it means that even though an early wrong answer was used to figure out the rest of the problem, little or no points will be taken off as long as the wrong answer was applied correctly to the rest of the problem.
Thinking he failed the calculus exam, Kevin was relieved when he saw BOPWA written on his test. Luckily, only a small amount of points were taken off for his "previous wrong answer" since he successfully applied the wrong answer to the rest of the problem.