I came out about being in a polyamorous relationship to a friend. Unfortunately he thought I told people about it, and because of asking a friend about her idea on it, she told everyone. Now I deal with people calling me a slut and coming up to my partners and telling them that I'm cheating or they should date someone who isn't a huge whore. I'm going into my senior year on Monday and I'm not ready to go back to the harassment and bullying. What can I do about this?
First off, I'm really sorry that this is happening to you. People can be cruel and ignorant to anyone who is different, especially when it comes to issues of gender, sexuality, or relationships. Young people especially can be really rigid when it comes to perceived notions of sexual morality and gender roles.
The best thing to do right now is to surround yourself with love and support. Check in with your partners - you're all going through this together, and be prepared to stand strong for each other. You can even come up with a script beforehand, so if someone tells your partner not to date you, they can have a prepared comeback: "Thanks for your input on my relationship, it has been noted" or "I don't really care what you think, I chose to date Gismeldo and I don't need your approval." Or just "that's so rude" or "that's not okay." Something that shuts down the conversation.
Try to let go of needing to change these people's minds - arguing with them is only going to make you feel miserable and targeted. It is okay that someone is out there thinking nasty things about you; they can have opinions that are wrong and walk around being wrong and that's their problem. I know it can be miserable and infuriating to be told "just ignore them" - I was bullied terribly as a child and a teen, and you deserve to be protected and supported. But fighting back on issues like this just becomes an ideological argument where you feel a lot more vulnerable and they get more opportunities to attack.
Stick with people you trust at school, and prioritize your own safety. If that means physically avoiding certain people, do that. If there is a teacher or other adult you trust, talk to them, but be prepared for them to be confused or dismissive; non-monogamy is not as widely understood as a potentially marginalized relationship orientation/identity and they might not be the allies you need. Try framing it as "I shared personal information with Zodielle, and now I'm being harassed by Lagron and Jezander." Share specifics about what they are doing and saying, and ask that teacher or other adult for anything you think would help you avoid it (like "Can I eat my lunch in a classroom with some friends" or "Can I avoid group work with these people").
If the harassment escalates from cruel words to anything else, keep documentation and report it. If anyone touches your body or your things, makes threats of violence, attacks your online spaces, etc. keep records and do what you need to keep yourself safe. But as long as it's just people saying you're a slut or a whore, try to take a deep breath, walk away, and remember that there is a huge polyamorous community who loves you, supports you, and has your back, and that those people are just flat incorrect.
Imagine that instead they're walking around saying "chocolate ice cream is literal poison and if you eat it you'll be vulnerable to psychic attacks from lizard people!" They're just misinformed, confused, and blatantly wrong; maybe annoying and potentially dangerous if they try to attack ice cream salespeople, but for now, they're just running around being obnoxiously stupid. Nothing they are saying is true - you are valuable, worthy, and perfectly deserving of loving relationships.