A group of young boys wearing track jackets stretch their legs on a gym floor as they sit in a circle.
The boys in their track jackets in the gym stretch their legs.
Boys are assumed to be young. For the boys to be able to stretch their legs on a gym floor, it is assumed they are sitting.
Group of young boys are boys, and wearing track jackets is a paraphrase of in their track jackets.
Boys "in the gym stretch their legs" is a rephrase of "stretch their legs on a gym floor."