。
She is a very good teacher.
Literal
She [topic-は] very good teacher is.
よい is the slightly more formal/literary counterpart to いい — they're the same i-adjective, but よい is preferred in writing, formal speech, and certain set phrases (~した方がよい, よい子, etc.). Both conjugate identically once you leave the bare attributive form. 先生 covers teachers, doctors, lawyers, and other professionals in Japan — anyone whose role earns ritual deference.