Larry was an officious a hole. He seemed to take it really personally that his vote count was questioned and may indeed have had the janitor go through the garbage to come up with gold.
"Chicken soup - with a beep straw."

I'm sure Larry was involved in Student Council in some way (the script on IMSDB.com says he's the outgoing president), so he wanted to make sure the vote counting was fair and impartial. And I think he was insulted that Jim kept insinuating that he made a mistake in counting, when it was clear he didn't.
Related question: when Jim reports to Walt's office the next day and finds Walt holding the two discarded ballots, both Larry and the janitor are there. I've always wondered if Larry went to the janitor and specifically asked him to check Jim's garbage can for the ballots when he emptied it? Or if the janitor found the ballots himself and reported it to Walt (because of the Chinese food incident at the beginning of the movie), and they called Larry to the office too because they wanted him to know they believed he counted correctly?

He is part of the process of helping up realize what an incredible douche Mr McAllister is. If Mr M was the good guy and good teacher he wants us to think he is, Larry would have accepted his word without question. Unlike the viewer, however, Larry knows Mr M is a douche so he is willing to call him out.

He was a lackey for Tracey and seemed to have it out for the teacher. I can totally see a teen acting like that today, they would probably ask to have the vote counting recorded by their iphone as well, or would secretly record it with a phone without permission looking for any irregularity to burn the teacher.