She is a shrinking violet who rarely speaks in class.
shrinking violet
idiomA very shy or timid person, who avoids contact with others if possible.
Origin
disputed originPossibly from the inconspicuous nature of the violet (genus Viola) flower and plant. The term appears to have been first used in the literal sense in the early 19th century.
Although he is a talented musician, he is such a shrinking violet that he refuses to perform on stage.
Despite her brilliant insights during private discussions, she remained a shrinking violet in large meetings, preferring to let others take the lead.