I started the thread, and I think this one takes the prize!
It didn't used to bother me to be honest. As a teen, I never liked sport and always thought I wasn't that good at it so avoiding it with an 'equipment check' was fine with me. Nobody brought towels any other day apart from the first PE lesson of the semester, but it was a pain as you'd have to bring two bags to carry rugby boots, shin pads, indoor kit, outdoor kit etc.
Anyone who didn't have everything would have an after-school detention and then have to bring the stuff to be checked the following week.
There were plenty of other petty rules too, but most of them have already been mentioned I think.
A few that really used to annoy at the time, but probably aren't massive in the grand scheme of things are...
The deputy head had a fascination with facial hair, or more lack of facial hair. The school actually had disposable razors and would force boys to shave if she noticed someone had facial hair. The secret for the lazy was to avoid her as nobody else seemed to care too much for that rule.
Blazers could only be taken off with the headmaster's permission. What he seemed (or maybe chose) to forget is that some of the portakabin classes were ten degrees hotter than any other, but woe betide anyone he or someone else in the SMT or a number of the more officious teachers found not wearing his blazer.
A bit different to others is that most people at my school had a nickname they were known as. Whether based on their first name, surname, an attribute or something else. Of course, you'd never call a teacher some of their nicknames as some were pretty harsh.
Merv the perv (no evidence of being a perv tbh), Psycho Price, Whisky Winters (used to stink of whisky at times and clearly an alcoholic thinking back), Spice boy, Roly-poly Rowland (a rather rotund man), Scouse C... probably shouldn't type that one.