That's how we were taught how to do it in school. We used to keep first 4-5 pages of every notebook blank to accommodate this. The first thing the teacher used to check was if the index was up to date. I guess it helped them too as assignments were easy to find. But then again latest home work was on the last page anyways .

Eventually some companies started producing notebooks with a printed index form. Those were more expensive and thus became status signals.