My guess - The girl you are refering to is called 
Kanzaki Urumi. (I have a few cels of her on my 
site)
My pick is the episode of GTO you are refering to 
is the first episode Kanzaki is introduced to 
Onizuka as she meets him climbing out of the park 
in a black dress.
She then fixes him up as you noted in the 
laudromat.
If you see some cool cels of her I would be very 
keen to have a look at them or even purchase them.