I have a contrasting POV here.
I was ok with group showers until I became aware self consciously, then I had to overcome my anxiety about it, which was not entirely successful. In the end I still have a little anxiety about my junk but it is because of the abuse not being naked with other naked kids or as I grew and went into the military, men.
Group showering under normal supervision is a healthy way to teach boys and girls to be secure in themselves. They get to see the range of normal and sometimes abnormal human forms and don't grow into adults who believe in ridiculous things about sex and body issues.
This will serve to help them function normally during sex and to let them know how the human body grows over time. Remember the little guy rarely doesn't turn into a normal guy before he is twenty.
Yes there are boys with anxiety over this and they should be shown compassion and given privacy. Eventually they must be told that they have to learn to lose that anxiety and eventually shower with the group even if it takes therapy it will help them in the long run to be comfortable in themselves and unashamed of their bodies. To me that would have been a wonderful thing to have had. I was told I had to overcome the fear but never given help in doing it, I was just thrown into the fray. I had to fake it and never did quite make it.
As Mark Twain once quipped, history may not repeat itself, but it does rhyme.