Updated: Sep 13, 2020
JK is an abbreviation of 女子高生 Joshi Kousei which means high school girl(s). The age of high school students in Japan are usually between 15 to 18 years old.
JC (じぇーしー) /jeishii
女子中学生 Joshi Chugakusei = junior high school girl(s)
JD (じぇーでぃー) /jeidii
女子大生 Joshi Daisei = university girl(s)