High school and college both last for a quadrennium. That's four years' time, even if, to some, it feels more like a millenium, i.e., a thousand.
Quadrennium is one of the words that specifically names a period of years––an annum is one year, a decade ten, a century a hundred, and a millennium a thousand. Quad- refers to the number four. In college campus architecture, a quad is a lawn surrounded by ivy-covered brick buildings on four sides.