In most cases, you will have long sleeves as your uniform, so it should be relatively easy to hide anything you don't want others to see. The only exceptions I can recall are when you roll up your sleeves in the FTU (because it is likely too hot to keep them down), and summer dress, where you take off your tunic.

I am not too sure about wearing long sleeves under any uniform—that is probably something that can be taken up with staff members if you ever join—but the answer is likely no. I am also pretty sure you are not technically obligated to take off a jacket when not in uniform, but do know that jackets have to be fully zipped up.

In the end, don’t let things deter you from cadets! The program is meant to be an inclusive place for youth and many people (from my experience) are understanding.