I can't really help with High School but here are some tips and pointers about Pubilc school(which i have attended my whole life):
1. There will be DRAMA! no denying it. It will happen. Just stay out of it.
2. Public school is not scary or hard or anything bad
3. Not having to wear a uniform(if you did) is really cool, you get to see everyones personal style
4. In high school you end up knowing everybody even if they are older or younger than you.
5. Sports are fun! If you want to go out i highly recommend it, especially if you are new, you make a ton of great friends through sports!
6. there are more people...lots of them
7. teachers really want you to be responsible so they will not remind you about assignments and stuff like that...my advice get a day planner or something to write down your assignments for each class
All in all, just have fun. public school is really no different than private, it is still school, there is still a dress code, there are still teachers and principals. so just have fun and enjoy it. make friends and try new things. you will have a lot of laughs and make a bunch of memories that will live on in your mind forever.