Umm as someone who had/has? the lowest self esteem, and I mean low to the point where I really believed everyone found me ugly/dumb and that I would never in my life get into a relationship, I wouldn't dare allow myself to think of anything regarding people because I was always an exception due to how not good enough I felt, and to this day I'm still kind of like that, the reason why I can maintain a strong eye contact and strong opinions, and stand my ground with confidence always, is mostly faking it, practice and being the one to compliment myself instead. Take a long time to meet your own eyes in a mirror and think of your good qualities, whether it's personality, looks or other.
This might not be the best advice but I find that being spiteful works to have confidence against those who target you. Think of how annoyed they'll be if you don't lose yourself to their criticism, take that thought with you to college and keep your head held high. Remember you're an invisible face in the crowd of college, everyone is so occupied with their own stuff, eventually even those people who target you have to get on with it and move on, and you'll be left behind permanently stuck in those past memories if you don't take care of yourself. No one can pull you down without your consent, sure you might still feel terrible but even if it's your pride, one small thing about yourself that you're proud of or wanting to win against them, you'll be able to look at people again. Think of yourself as the only one who can protect your own self.
I'm talking from a decade of experience with this, there are literally so many ways you can build up your confidence, fake it or maintain it through time. Good luck op, don't listen to those staffs, they're probably envious you had confidence in the first place. <3