。
She thought back on her student days.
Literal
She [topic-は] student days [object-を] recalled.
学生時代 ('student days / one's time as a student') is one of a small group of fixed time-period nouns built with ~時代 (literally 'era / period'). Others include 子供時代 ('childhood'), 若い時代 ('one's youth'), 江戸時代 ('the Edo period') — the suffix is productive across both personal life-stages and historical eras. 思い出す ('to recall, remember') is itself a compound: 思う ('think') + 出す ('bring out') — 'to bring [a memory] out of one's thoughts.' The を marks the time-period as the object of recollection, a slightly stretched use since you don't literally remember a span of time but its contents — Japanese is comfortable with this shorthand.