By far not the stupidest, but it's one that's coming to mind.
The school was in a poor area and had a mandatory school uniform. One of the rules was that for boys, "school shoes" must be worn, not "boots". In many cases, the distinction is obvious, but in ambiguous cases, the distinction came down to how high up the shoe/boot went. I think they defined a length that was the boundary.
What's silly though is that this length was such that if you were wearing regular school trousers, it would be impossible to discern whether it was a shoe or a boot. At uniform inspections, they would literally have people pull up their trousers legs enough that they could see the top of the shoe/boot, and measure it with a ruler. Inspections were usually overseen by a senior member of staff (not the same one each time).
My brother was sent home from school because his brand new school shoes were 0.5cm too high and were therefore boots. He wasn't meant to return until he'd replaced them, but my mum called the school and went nuts because she couldn't afford to replace them for such a stupid rule. They "made an exception" in this case.
Girls schools have the same deal with skirt length. They make a fuss over it because they know the kids will rebel over a stupid rule like that, instead of the kids rebelling by doing drugs.
Similar story. Few years ago in a previous life, I was a teacher. Our administration would make an announcement in the morning that we had to check uniforms in first period and send non-compliant students to the office. Kids were generally in uniform, but a lot of the rules were nitpicky. One of the uniform rules was that students had to wear a certain color sock of a certain length. I don't care what socks kids are wearing so never checked. They're under their pants. Our assistant principal would stand in the hallway and check uniforms. When he found kids out of uniform, he would figure out who their first period teacher was and send a nastygram. Imagine living a life where you allow your blood pressure to rise because of the kind of socks on somebody else's feet.