This is a really difficult area, and has a lot to do with culture. British boarding schools are no longer permitted to have any sort of communal shower, so even though my cadets are all boarding school pupils they are still very uncomfortable with this sort of shower arrangement on camp. Yet when we visited an American boarding school last year, it was all communal showers and the Americans thought we were weird for being concerned about it. Yes, our pupils mostly showered with underwear on, this does seem to be a common response.
But - cadets are absolutely right in the target age for being self-conscious about body changes (and speaking personally I HATED communal showers at boarding school in the 80s, not just because of the maths teacher who lingered longer than necessary when he came in to tell us to get a move on). I can see that you are seriously considering whether or not to go on camp based on the shower provision. I don’t know anything about your personal situation - if you have more than usual reasons to be worried about this then please do speak to your Sqn staff about it, as it is a welfare issue. But if you are just the average teenager then the advice above to grit your teeth and put up with it for the rest of the camp experience is good advice, and sooner or later you will find yourself in a similar situation as an adult and will be able to cope because of your cadet experience.
Personally… one of the most awkward experiences of my life was sharing a communal shower with my newly-appointed ex-Army School Staff Instructor on a mountain leadership course when I’d only known him a couple of months, he is seriously fit and muscley (unlike my dad-bod) and he is covered head to toe in tattoos. But of course as he is ex-Army he took it in his stride…