That was when I was a first-year in high school, so seventeen years have passed.
Literal
That [subject-が] high-1 [genitive-の] time [copula-だ-because-から] 17 years [subject-が] passed [polite-past-ました].
高1 ('high 1' = first year of high school) is the common abbreviation for 高校一年. Combined with the だから ('because'), 'that was in my first year, so...' sets up the temporal calculation. 経つ ('time passes') takes temporal subjects: 17年が経つ ('17 years pass'). A nostalgic reflection.