You see, English is a bit more complicated than having just people, things and places as nouns. If you dive into the dictionaries, you will easily find that a quality, without a link to someone in particular, can be a noun. Same goes for character traits, occurrences and much more. The adjective “penurious” is no different. See, if we forget about the links of an adjective to its noun, we will get a trait, a fact, that is a noun itself. Dictionary allows us to use both penury and penuriousness as nouns, though the first one I would call slightly more popular.
If we shift the facts around a little bit and assume that penurious student can be called a student that is used to penury or experiences one at the moment.
That student can also experience happiness, sadness and plenty other occurrences and feelings and all of those would be nouns while we try to describe them.