Veronica doesn't investigate those who stand in the way of her personal interests until she finds enough dirt to destroy them, the way Ratner chose to do to Veronica--if she did, she would have investigated Angie Dahl and have been the one to expose the doctor's note scam Angie had been abusing to postpone exams to her benefit, not Keith. Veronica only uses her investigative powers in those cases when a) someone asks her for her help or b) it's part of the larger corruption of Neptune--like, she notices evidence of a cheating ring that serves to keep 09ers on top academically above anyone else playing fair.  She doesn't look at someone doing better than her academically and assume because they're rich or an 09er that cheating must be going on--she doesn't even go after Angie, who is everything she despises about Neptune High and knows doesn't deserve the higher grade for something she paid for, to find something to get her out of the way.

Veronica doesn't even rat on people she knows cold cheated, even when it's likely they won't get in much trouble for it anyway, like with Logan and the extracurricular mayoral contest. And in the one case where she does expose a cheater, it's because a) he's coming after her specifically in a scholarship contest that's already tipped against her by virtue of her not having 09er money to spend on tutors/other advantages and has already tried in the episode to pin his own part of his crimes on her--and while she may not be about looking for an excuse to tear anyone who stands in her way down, she does the revenge thing well, and b) she gave him the option of either owning up to what he did and facing fair punishment or the option of getting away with cheating with way worse potential punishment. She let's him decide after confirming the fuck out of her theory that he's the cheater to her own satisfaction.

Ratner looked at Veronica and deemed she didn't deserve the grades she was getting with a) absolutely no demonstrated evidence that Veronica was stupid or dishonest and b) decided to bring her down without even bothering to confront her about it--which no doubt Veronica would have done before making any move to expose him if their positions were reversed. I suppose you could make the argument that Ratner didn't have Veronica's resources to check it out--but then, what does basis does an argument claiming Ratner has anything in common with Veronica other than a bias against the wealthy and snotty? Nothing.

Like, people wanna say that season 1-2 Veronica is the kind of person who would rather suspect people excelling academically beyond her were cheating, instead of the hardworking, hyper-focused girl who smashed the competition by virtue of making sure her own work outshone anyone else's? No, not Veronica's style.  Ratner's the one who saw girl + rich boyfriend + doing better than me in class = obviously cheating.  That's a commitment to finding the truth and exposing corruption ala Veronica Mars...how exactly?

Veronica's no rat. That's her code of honor. And she doesn't assume that anyone doing better than her academically is automatically cheating because whoever they are, they're not deserving of the grade they got for their work (which is what ratner did with Veronica, period).