。
She was standing among the boys.
Literal
She [topic-は] boys [possessive-の] among [in-に] was-standing.
少年たちの中に ('among the boys') uses 中 (なか, 'inside, among') to place her within a group. 立つ (たつ, 'to stand') in the ~ていた progressive past sets a scene: she was standing there at that moment in the narrative.