Would the average Frum boy be “over” (transgress) on histaklus (or at least find it difficult not to be) if a Frum tzniusly dressed girl were to walk in front of him?
I have no idea, but it shouldn’t change your actions. The boy the the one who is mechuyav not to stare, you are not. I would hope that most religious Jews would turn aside or look down and not stare.