。
She's been at the top of her class the whole time.
Literal
She [topic-は] until-now continuously class 's top is.
首席 ('top, head, foremost') is the formal Sino-Japanese term for the number-one student in a class or graduating cohort. In Japanese academic discourse, it's a real distinction — 首席卒業 ('graduating at the top') is a phrase used in profiles and CVs. ずっと here is the temporal 'continuously, all this time,' tracing her position from some past start point to now. The polite copula です fits the slightly elevated register of 首席.