I am most likely headed to my state flagship school in the fall, and am in love with just about everything it has to offer. However, there’s one thing that is making me incredibly nervous: I’d be going with at least 100 of my high school classmates. This doesn’t seem like a huge issue when the college’s student body is around 30,000, but everyone from my school wants to lIve in the same dorm, rush, and many even share my major. I enjoyed high school a lot but really wanted college to be a fresh start where I could come out of my shell a little more. I feel like I’m going to have a hard time doing that seeing people who know the old me everywhere. I’m also thinking about rushing, but I’d be scared doing it with all of them. What do you think about this situation? It’s hard because this is my only issue with the school. Sorry for the long post.
Don’t worry about your old high school classes. You are going to a big school. You likely won’t see many of them, and even when you do, you don’t have to spend time with them or talk to them if you never want to. Students from my high school attended my school and I barely ever saw them (some of them I saw once in four years, others I saw a couple of times). When I did see them, we’d maybe say hi and then keep going with whatever we were doing. They will have no impact on your life, unless you want them too.
Even if they also rush, it’s not going to be nearly as many people as you think there will be rushing with you. All 100 students aren’t going to rush with you (and presumably about half of them are going to be the opposite gender as you anyway), and even if many of them do, you don’t have to stick with them. You can talk to other people and hang out with them, rather than sticking with kids from your high school.
Don’t let where your high school classmates go affect where you go to school. They won’t affect you, unless you let them.
You can go to a school like that and completely immerse yourself with HS friends. My son is in his 5th year and does not have 1 single friend who wasn’t from either our HS, or our other neighborhood HS. On the other hand, I went to a big state flagship and can tell you that they are so big that you can totally avoid HS people if you want. You will still see them occasionally on campus or even in a few classes, but finding a new group of friends is really easy.
Start by not living in that dorm and by finding an out of state kid to room with.
Right now they are saying those things because HS is all they know. It seems safer to them right now.
I would not do anything to try to stay with or avoid your classmates. Don’t sign up with one of them as a roommate. Go with whatever dorm is chosen for you or pick another one and just tell them “I couldn’t get into that dorm”.
You may see them at Rush, but you don’t have to go with them specifically. If they take your major, so be it. Usually at first there will be many sections of intro classes and it is unlikely you will be with all of them.
So really I would say to them “yeah, it will be great to see you around” but then make your own plans which may or may not include them.
Lotsa students try to remake themselves when they go to college. I’m sure some are successful.
Join a club that they aren’t in and hang out with that club! My HS friends that go to the same university can be found in the same corner everyday and I generally only stop by every now and then to say hi.
As a reserved person, starting new is wonderful. You don’t have to start out telling everybody everything about yourself and as they get to know you better for the person you are / over first impressions you can surprise some people 
What state school?
I feel a bit sad that some of the advice is to actively choose a different dorm, etc. I’m not saying it’s wrong advice (!), but ideally you shouldn’t have to go out of your way and make choices contrary to your own preferences just to find your own path. I think you understand the challenge you face, and only you can decide if you will be comfortable with it.
Fwiw I can understand your dilemma which is, perhaps, why I feel for you. At a restaurant on the way home from visiting colleges, my dad made up a pro/con comparison (on a napkin, which I still have!) of BigStateSchool vs NeighboringBigStateSchool. BSS had many pros and only one con…it was the BSS. I just didn’t want to go where everyone else from my hs was going. I didn’t go there. It was the right decision For Me.
Best wishes!