Meaning of “JK”

“JK” means a high-school girl as a Japanese slang.
This word is an abstraction for “JoshiKousei(女子高生)” which means a female high-school student. The pronunciation is JK [dʒéi k].
This slang has been in use for over 10 years, I think. We still use this.

“JS” means a elementary school girl and this word is an abstraction for “JoshiShougakusei(女子小学生)”.
“JD” means a female university student and this word is an abstraction for “JoshiDaisei(女子大生)”.

Additionally, over a decade ago, JK had another meaning as a slang.
The meaning was “use common sense”.
In this case, JK was an abstraction for “JyouKou(常考)”. “JyouKou(常考)” is also a old slang and this is an abstraction for “Joshikitekini Kangaete(常識的に考えて)”.
“Kangaete(考えて)” means “think”, “Joshikitekini(常識的に)” means “in a common-sense manner” kind of.
When using this slang, it is typically placed at the end of a sentence.
Like this.
それくらいわかるだろ、JK！(sorekurai wakarudaro, JK)
*It means “You should understand something like that, JK!”.

Anyway, we don’t use this slang anymore, so if you find this “JK” word in Japanese, it means a high-school girl.

Example Conversation

A: It’s almost time to stop wearing swimsuits for the summer.
B: Why?
A: I’m turning 30 soon, and I don’t want to be compared to JK…
B: Got it…

A:もうそろそろ夏に水着きれないかなぁ
B: なんで？
A: もうすぐ３０歳だし、ぴちぴちのJKと比べられたくない…
B:なるほど