Japanese Wikipedia has an article on:

コギャル

Wikipedia ^ja

English Wikipedia has an article on:

Kogal

Wikipedia

First appeared in the early 1990s, especially after the term’s popularisation in the 1993 Late-Night TV Special “Za kogyaru naito” (The Kogal Night), though other theories regarding the term's derivation exist.

A likely derivation is attributed to teenage girls dressing as ボディコン (Bodikon women) at discothéques in the early 1990s, therefore being a slang abbreviation to indicate the many young women who was underage in these discos.

• From 高校生 (kōkōsei, “high-school student”) + ギャル (gyaru, “gal, young woman”, originally referring to someone not a minor).
• From 格好 (kakkō, “appearance, seeming”) + ギャル (gyaru).

Another possible derivation is attributed to the mass media.

• From 子 (ko, “child; young; small”) + ギャル (gyaru).

The gyaru portion in all of these is a borrowing from English gal.

• IPA^(key):

コギャル • (kogyaru) 

1. a teenage girl participating in the kogal subculture in Japan

Kogal subculture is a form of the ギャル (gyaru) fashion subculture and peaked in the mid-1990s. Kogals were typically described by the Japanese media as tending to be obsessively trend-conscious.

• ボディコン (bodikon, “bodycon”)
• ガングロ (ganguro, “ganguro”)