。
Her English grades are good.
Literal
She [topic-は] English [genitive-の] score [subject-が] good.
A textbook double-subject (XはYが) construction: she (X) — when it comes to her English score (Y) — is good. This is one of the most common shapes for ascribing properties of a subdomain to a person: 私は頭が痛い ('I have a headache' — me, head-wise, hurts), 田中さんは目が大きい ('Tanaka has big eyes'). よい is the literary/written variant of いい ('good'); both forms coexist, with いい dominant in everyday speech and よい preferred in writing or set phrases (運がよい, 仲がよい).