Nah…those cases are pretty much tarud b’melachto; it would be permissible even in front of Jews (he is focused on making a production/class and not on his students dress. Of course, this applies only during the class – before, after & any social connection and the like would have normal rules of tzinius). This is why we permit women to see male doctors in all settings, even if a female is available, and there is no pikuach nefesh (say, elective surgery & the like- a different schmooze as well) . Presumably, this would apply to lifeguards as well. Still, women presumably are more comfortable around a female staff.
I wonder though, about other attendees (not the instructor) of the class. They presumably are not tarud b’melachto, and may merely be attended the class for it’s fringe benefits. Presumably, if it is not an all-female class, it would be problematic.
Of course, even the instructor, this means you can assume his mind is elsewhere. Of course, if it’s clearly not, based on comments & actions – then it would be forbidden.
Again, I see no difference between Jews & Non-Jews. Jews who are professionals in these areas are permitted to interact with immodest women, based on the heter of tarud b’melachto. Of course, a Ben Torah should choose his profession with brains. I’m not sure if women’s clothing designer is what a Ben Torah does for parnassah (try astrophysics. Much more fun. And you get to play with explosives. And no women – except in astronaut suits, which are pretty tzinius. Meah Shearim is actually going to bring in a few.)
As far as pants…that’s a different schmooz. Di L’Tzara B’Shaato.
For all practical applications, please speak to your LOR. This is theory only.
And you can see the subtle & slippery slope here. Bnos Yisrael who keep tzinius in all settings around men are certainly in the geder of ??? ????.