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There's no way she had good grades at school.
Literal
She [topic-は] school [possessive-の] grades [subject-が] were-good [can't possibly-はずがない].
~はずがない ('cannot possibly / there's no way') is a strong epistemic negative — the speaker is asserting that the proposition is impossible based on what they know. はず ('expectation / supposition') becomes negated existentially with がない, so literally 'an expectation [that this is the case] does not exist.' Layered with the past 良かった, the meaning is 'there's no way she had good grades' — strong skepticism about a past state. 成績 ('grades, marks, performance') is the standard word for academic results. The whole sentence carries a tone of incredulous correction: someone has just claimed she did well, and the speaker is pushing back hard.