First off, be yourself. Don't try to please everyone...because you can't...and then you won't be happy because you've been trying to please everyone that you can't. Second, the shirt deal...I, personally, am not sure it's that big a deal, if the dress code isn't being enforced I'm not sure I would worry about it. If a person in authority tells you tuck your shirt in, then tuck it in and keep them tucked in. I know my daughter always comes home with dress code and about a week or 2 into the year teachers aren't even enforcing them. Probably the biggest issues are girls revealing too much skin and guys wearing those DISGUSTING low riders so you can see those snazzy boxers they're wearing and anything that could be considered gang-related.
Also, keep in mind that when you go through nursing school you have to do all the stuff that you don't want to do in order to graduate so you can move on to do the stuff you want to do. I knew at 16 I wanted to be a nurse...I was a CNA & was wiping tushies even way back then. Sometimes you have to take a little bit of icky to get to the really good stuff.
Best of luck with your new school and future nursing school.