I thought that "peoples"(non possessive) wasn't a word!?

The word used in the video is "[pupils](https://en.wiktionary.org/wiki/pupil)", which means students. However, "peoples" is in fact a word. It's the plural of "people" and can be used when referring to multiple races, groups or nationalities.

I heard ‘pupils’, which is a somewhat fancy word for ‘students’ though ‘peoples’ does get used sometimes as a plural counting more than one group of people, usually grouped by something like their nationality.

The word is "pupils" meaning students

Grammaticality is a word you should familiarize yourself with. If someone uses it and you understand it, it's correct. The only thing you should concern yourself with is stuff that you don't understand what they mean.

They're saying pupils which means students. Peoples can be used that way though but it usually means groups of people based on culture, ethnicity, or nationality, eg "the Anglo-Saxons, the Danes, and the Normans were three peoples who are inextricably tied to how the English language developed."