。
She goes to an all-girls high school.
Literal
She [topic-は] girls'-high-school [to-に] is-commuting.
通っている uses ~ている to express a habitual, ongoing action — she regularly commutes to this school. 女子高校 (all-girls high school) is the formal term; the casual abbreviation 女子高 (じょしこう) is far more common in everyday conversation. Single-sex schools remain a visible part of Japan's educational landscape, particularly at the private secondary level.