"High school students"
No, this simply reflects the sometimes seemingly arbitrary nature of abbreviations. 高校 is short for 高等学校（こうとうがっこう）.
In Japanese, many things get abbreviated upon hitting the 4-character threshold. While this is not a rule per se, it is very dominant and 小学校・中学校 don't quite meet the threshold.
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.
It's a pluralizer. Added as a suffix to the end of a variety of nouns, it serves to indicate multiples.
子供 child; children
先生 teacher; teachers
Aside from pronouns, Japanese nouns are usually neither specifically singular nor plural, but context will usually suggest whether a noun should be considered singular or plural. In cases where it is not clear or emphasis is desired たち can be added.
Hmm, but should it have really? I know google translate isn't the most reliable, but it translates "高校性たち" as "high school sex" (the たち gets ignored in terms of meaning), since apparently the first reading for 性 that it suggests is "sex" XD Even a translation site like jisho says that that kanji is related to sexual stuff (it can also mean gender, which was #2 in google's possible readings too). So yeah… 性 might have the same pronunciation as 生 in this sentence, but it certainly seems to convey a different meaning in writing :P
Interestingly, google has slightly different voice inflection for the two versions of the sentence, and I've recently heard that that can clarify meaning (or maybe more accurate to say that there's a "correct" intonation for words) when it comes to Japanese words that are pronounced the same way but have multiple meanings. I think it's mainly just one of those things that can possibly come off as an accent if you say something with the "wrong" intonation though (you're not likely to confuse a native since the pronunciation is technically the same after all, so there's little room for contextual misunderstanding in a typical sentence).