Should I pour spoiled milk on my sisters bully?

IDK how to really write this so bear with me. My sister and I go to this preppy religious high school, and since she's a part of the LGBTQ she's been getting bullied. Not just mean comments, but she's been called shit like f@ggot girl, a furry, etc. It obviously bothers her (so much she's thinking of dropping a class) but she won't let me or our parents do anything about it. It pisses me off so much, but I don't want to cause more trouble for her. I was thinking of coming up to the bully and pouring spoiled milk on her head or something and calling her a bitch. I know it's dramatic but I can't just sit by and watch my sisters mental health just suffer. The only problem I see with this plan is that I could get suspended (or worse, expelled) for doing something like this since I go to a VERY strict school. I'm wondering if anyone has any suggestions on what to do.