[Hiraeth.]

Your mother is right. This is not gym attire one bit. That's great your friends do that, but they're exposing themselves (again figuratively and literally) to people that would wish to do nothing but harm upon them. I'm by no means criticizing your freedom of expression, but teenagers these days really don't seem to care how they dress and what sort of people that'll come across them.

Sports bras would fit differently and would not expose as much cleavage. What you're demonstrating does not indicate that it'll offer support when you're doing cardio or weight lifting. You need appropriately fitted clothes that offers support to the breasts rather than draw attention to it.

Your mother is absolutely not a prude. She is right, she is concerned about you, as mothers should be when their daughters go out dressing a certain way.