My go-to, especially when I know I'm not gonna see this person much outside of a specific context, is just to tell them that I already have a boyfriend/partner back in my hometown (or just wherever I'm at, depends on who's bothering me). People tend to back off after that, but idk if it works in your case cuz it's a lot of people.
Good luck! Hope this shity situation gets better :(
And, as others are saying, please tell a teacher, if you can/want. They can help intervene, mediate and defend you in a situation like this where it's several students practically ganging up on you, pressuring you into something and making you feel uncomfortable.