。
She's completely changed since her high school days.
Literal
She [topic-は] high-school-time since continuously changed.
~以来 ('since, ever since') is a formal time marker attached to a noun or verb-past form, fixing the start point of a continuing situation. 高校時 is a slightly clipped form of 高校時代, common in writing for compactness. ずっと doubles down on the sense of continuous unfolding — 'all that time, she's been a changed person.' 変わる ('to change') in past tense gives a settled-into-changed reading: she became, and remains, different. The whole sentence describes the durable contrast between past and present.