。
She and I are in the same class.
Literal
I [and-と] she [topic-は] same class is.
The 僕と彼女は construction uses と to conjoin two nouns into a single plural topic — 'I and she' as a unified subject. The sentence then treats this composite topic and states a shared attribute (同じクラス). A very natural way to say 'we're both in X' where X is a shared category.