、。
In high school, she was good at math.
Literal
High-school-era, she [topic-は] math [subject-が] good-at was.
Note the word order — 高校時代 fronts to set the temporal frame before the topic 彼女は. Japanese is flexible about adverbial fronting, and time-frames at the start of a sentence give a 'in [time], [statement]' feel that's slightly more narrative. 得意 is a na-adjective like 好き — the thing one is good at takes が, not を. Compare: 数学が得意, ピアノが上手, 料理が好き — all stative predicates with が-marked subjects.