Is it me or was my school the only one where girls wrapped themselves up in skipping ropes and behaved like horses, or did this madness exist elsewhere? The guys at my school never really understood what was going on.

Actually, the girls were indulging in experimental lesbian fantasy, as this frame from "The Adventures of Gwendoline" by John Willie illustrates.

It happened at our school too. In junior school there was a whole 'riding school' thing where we brought in riding crops (or if you were a poor kid and didn't have a pony, you used a stick) and jumped over branches propped between trees, hitting ourselves with said implements and pretending to be on horseback. Or perhaps simply preparing ourselves for future bondage sessions. It was run by the most popular girls, and you could only join if you jumped a set course with sufficient style. Let's hear it for all-girl catholic schooling...

Oh absolutely. When the monitors first caught us at it, they sternly told us we could play with the jumpropes only if we stopped "lassoing" them around each other's necks. With this logical alternative taken away, we then became extremely creative in bondage: arms, legs, waist, you name it. Being tethered to a tree was the coup de grace. Luckily a hill on the playground concealed our activities, but when a girl who hadn't come back to class after lunch was discovered voluntarily hogtied to a small spruce... Well, you can guess that they didn't let ANYONE check out jumpropes after that.

Our riding school, in which the best girls wrapped jumpers around their waists and were ridden by a girl holding the arms, came under attack. Our innocent adventures were legendarym until two girls, who didn't seem to harbour any equine aspirations whatsoever, called themselves Trixy and the Magic Mule, and attacked the poor, popular horses. Obviously not in with the Cheshire Set.
We solved this problem by throwing stones at them, and calling them "common".