See my first answer too. Some schools are requiring the full physical at that age already, some aren't. If you have a physical form, it will say on it how far they are going to go.
If you are going to have the full exam, everything comes off - bra and panties as well. You wear a gown so you are somewhat covered but you will be mostly naked as they check inside.
If you aren't having the full exam, you will probably get to keep your bra and panties on - or you might get to keep all of your clothes.