I think we actually were "required" (not sure, because I totally avoided the changerooms, as mentioned above), but I still didn't do it. There's no way in hell I'd let a teacher force me to get naked, yk? And, I still can't imagine any reason for a teacher to be standing there while I stripped.
My daughter is almost 14 and last year when she entered JH I told her straight out "If they make you shower you have my permission to refuse" because she would have rather walked in front of a truck than get naked in front of her classmates just based on her very private personality. I was ready to go head to head with the school on it if I had to. Luckily someone came to their senses when they built this new JH because they don't have to shower and the showers have curtains- one shower one kid at a time.
And on the topic at hand, I think in general in America (culturally big generalization here) it is probably odd to shower WITH your grandkids, esp both grandparents. I would have been humiliated and felt sexually abused by it if it had happened, because like I said above I never questioned adults and wouldn't have felt like I could have refused. I remember parents of my friends after going swimming insisting that I "just change it's just us girls" and feeling violated by it and trapped with no choice over my own body.
Some of us (I've been this way since all the way back and my mom is basically a nudist so it wasn't upbringing- more like personality) just aren't comfortable with the everyone get nakey together stuff. I'm actually totally fine if anyone else wants to be naked- but I don't want to. It's not gross, it's just well IMO uncomfortable and it makes me feel bad inside. I do not like it.