That's how all education works. It's the spiral model of teaching. In one grade you learn a bit of this and a bit of that, then the next year you retread, and flesh all those things out by adding more depth and complexity. Rinse and repeat every grade.

What would the alternative look like? Should a foreign language course spend three years on Nouns, just to make sure they're comprehensively covered, before you ever see your first Verb?