。
She's at the top of the class.
Literal
She [topic-は] class [in-で] top is.
トップ is a loanword from English 'top,' but Japanese tends to use it specifically for 'top of a ranking, leader' rather than the spatial sense — for that, 上 or 頂上 would be more natural. クラスでトップ ('top in class') refers to academic rank, similar to 首席 ('class valedictorian, top student') in more formal contexts. The で marks the scope of the ranking — 'within the class.' Loanwords like トップ often slot into idiom-like positions where a Sino-Japanese alternative exists but the loanword feels more contemporary.