。
She's a serious student.
Literal
She [topic-は] serious student is.
真面目 (まじめ) is a na-adjective meaning 'serious, earnest, diligent.' It's one of the most common personality descriptors in Japanese, covering both the positive sense of being hardworking and reliable and the sometimes negative sense of being too rigid or humorless. In a student context, it's almost always positive.