Long Uniform Skirts in Japan

I've searched using the terms 'long uniform skirts japan' 'lengthening uniform skirt japan' 'yankii uniforms' 'sukeban uniforms'

My timeline? Japan around the 1970s.

My question- how did they lengthen their school skirts? Also, if you have any other details about the dress of the sukeban or yankiis of that era, I'd love it.