In A Separate Peace what happens to Gene's grades after he flunks the test?Do you think Gene would have become such a "good student" if he hadn't been so angry at Finny?
In chapter four, after Gene flunks that first test of his life, he really buckles down and studies hard. As a result, he does quite well. He focuses, studies, learns, and even does better than Chet Douglass, who loves learning so much that he gets distracted by it. Gene says of his efforts that
"I was more and more certainly becoming the best student in the school...I redoubled myself."
So, Gene actually does quite well after the bad test, and feels good about it.
You could say, possibly, that it was Gene's false delusion that Finny was trying to undermine him that drove him to such great academic heights. Gene thought that he was in competition with Finny, so, worked harder. He wasn't going to let Finny win. However, there isn't much textual evidence of that conclusion; instead, there is more focus on how Gene thought that Chet was his main competitor. More likely, Gene's suspicions of Finny released him from feeling obligated to tag along all of the time; it broke some of Finny's power over him, and that did indeed allow him to study harder. I hope that those thoughts helped; good luck!