I have never seen a betta fight, and I have a feeling I never will. To answer your question, if a kid, or kids at my school, (10th-12th, bound to do stupid things anyhow...) pulled a stunt like that, I can guarantee that I would do more than something about it. I sincerely hope that this was a theoretical question. If it is not, and this really happened to you, confront a teacher or principal. Not only is this animal abuse, but you should not fear standing up for what you believe in, no matter what. Teachers or principals should value this as well, and can probably deal with it while keeping you anonymous.
Really though, I doubt you would get teased as it is. If you are a girl, it is more socially accepted to have strong passion and empathy for animals. (not saying this is true in all cases, it is a stereotype that a lot of people, especially school aged kids, believe.) And if you are a guy, I promise you that by standing up for weaker creatures, you will become more popular with girls. No one will make fun of you for that, either.