I am also a struggling college student trying to stay primal through social situations. It really is tough... I haven't always been able to avoid pizza, beer, cookies, chips, etc. every time, but in the grand scheme of things, moderation is key. Usually for me, whenever I would "cheat," I would basically say, "well eff it" and then continue eating poorly the rest of the night. If you are going out in social situations and you do want something, just make sure you only have one or two things. Moderation, moderation, moderation. You could also try eating your fill with a primal meal before the social gatherings so that you are less hungry and won't even want to eat what is offered. And just a simple "no thanks" can work wonders.
My roommate definitely makes snarky remarks whenever she sees what I eat... I try to ignore it. I tried explaining it to her once but she wasn't really getting it, so I stopped trying. Once you start seeing the health benefits, it won't matter what other people say because you are doing it to better yourself. Or just tell them you are gluten intolerant and they'll usually leave you alone, haha.
Best of luck with your first 21 days!
Two important questions to ask yourself every week:
Did I run faster and farther this week than I did last week?
Can I lift more weight and do more repetitions that I did last week?