。
She's the happiest in the class.
Literal
She [topic-は] class [in-で] number-one happy is.
A simple superlative with 一番 ('number one') prefixed to the na-adjective 幸せ. The で here marks the scope of comparison — 'within the class.' Note how Japanese can drop の中 (the 'within') and just use で for a tighter, more colloquial scope marker; クラスの中で and クラスで are both fine, with the latter sounding a little more natural in spoken contexts. The polite ~です gives the sentence a measured, schoolroom-like tone.