"High school students"
Duo we need the on AND the sen readings for all of the kanji. And teach us the kanji, AND the hiragana.
Can't 高校生 be interpreted as plural? As in "high school students are fickle."
Yes. There is nothing specifying singular or plural with Japanese nouns. It must be gleaned from context. Your sentence example would provide ample context.
There is no plural specifically indicated in the base noun itself. That is why the plural indicator たち is added.
わたし - Me/I
わたしたち - We/Us
ぼく - Me/I
こども - Child/Children
こどもたち - Children (definitively plural)
とり - Bird
とりたち - Birds (definitively plural)
*As 'Me/I' is by default a singular construct it will always be assumed that without adding the plural modifier たち it will be one person.
This seems to be more of a 'how' question than a 'why' question. Asking 'why' is likely to lead you into a rabbit-hole that you don't want to go down.
Just take it for granted that Japanese does not use plural inflection. Japanese nouns are by their very nature 'zero plural'.
We have these in English as well: Sheep, fish, shrimp, etc. Yet, we somehow make clear (from context and quantity modifiers) whether we are talking about one, many, or a specific quantity.
'The sheep is', 'The sheep are', 'Some sheep', 'Many sheep', etc.
If this answer is unsatisfactory I would suggest revisiting the question after learning the language, if you still need to or desire to. If you can't wait, there are online resources that might better address your concerns.