KatiePie:
The slightest movement revealed knickers.
I remember it well as I went to a liberal co-ed SecMod. 1968 saw the shortest skirts, where the girls would hike the waist bands up to the point where if they were just one step above you on the stairs you saw their knickers. 1969 saw long pencil skirts come into fashion.
Girls never got smacked or caned at my school though. Some spent half their life in detention though.