。
She's better at music than anyone in the class.
Literal
She [topic-は] music [in-では] class [of-の] anyone [than-よりも] is-superior.
音楽では fronts the field with a contrastive では: 'in music specifically, as opposed to other subjects.' 誰よりも ('than anyone'), with the universal-quantifier よりも, sets up an absolute superlative — better than every other member of the class. 優れる carries a slightly elevated air compared with the everyday うまい ('skilled, good at'); it's more often used in formal evaluations and recommendations.