。
She's by no means an ordinary student.
Literal
She [topic-は] never ordinary student is-not.
Two emphatic moves combine. 決して... ではない is the 'absolutely not' frame — denying a category outright. 平凡 ('ordinary, mediocre, run-of-the-mill') is mildly negative on its own — calling someone 平凡 implies a lack of distinction. So negating 平凡な with full force of 決して produces a strong endorsement: 'far from ordinary, she's exceptional.' This pattern (決して~ではない) is a common way to issue praise by ruling out a contrasting label.