At my school at about the same time, girls wore red or green pin-striped summer dresses, and they were very short indeed. Most girls wore a belt which aided the prevention of them riding up, revealing knickers, but had the advantage of tightening the dress around the bottom area. The temptation to smack a bottom in these tight, very thin dresses was intense.
When I was in the 6th form we didn’t have prefects but we were expected to do some duties. One of these was patrolling the corridors at lunchtime just to check on things. Pupils were expected to be outside during break times, and I came across a group of fifth-form girls still in a classroom. I requested them to go outside which they appeared to do. However, about ten minutes later I returned and two of them were still indoors.