I wore a uniform from age 5 until I graduated at 17. In primary school it was a blue pinafore-style dress, white collared shirt or turtleneck, navy blue tights or white socks, and brown lace up leather shoes. If cold, a navy jumper or school logo sweatshirt was allowed. No logos (besides the school logo) of any kind allowed. No jewelry of any kind allowed. No nail polish and definitely no makeup allowed. Our gym uniform was a plain white shirt and red school-issued athletic shorts. Once in middle school and high school (age 11-17), the pinafore dress was still allowed but we were given the option of also an a-line blue skirt (same fabric as the dress), and from November-February, we also had the cold weather-appropriate options of a wool kilt (headmistress was Scottish) or navy blue corduroy trousers. Also, in addition to brown lace up shoes, we could wear predominantly white trainers (nothing flashy). Still no logos, minimal, understated jewelry, minimal makeup, no nail polish or too-long nails.
At any given time, I owned two uniform skirts, five uniform shirts, one navy jumper, five pairs of navy tights, five pairs of white socks, and one pair of school shoes. It all fit in one drawer and I got dressed every morning in a matter of minutes. For weekends, I had usually a couple sets of "play" clothes and a couple of dressy outfits. None of these clothes were replaced until they were worn out or too small. There was never any pressure about what to wear to school - it was easy. I loved it!
It also taught me the lesson of right time, right place, right clothes. Play clothes are for play. School clothes are for school. Same as for adults, work clothes should not be the same as gym clothes or hiking clothes or clubbing clothes, or gardening clothes, etc.
My school was all girls, so there was no boy/girl differentiation; however, we had a huge range of incomes, and the uniform was very much an equalizer. I didn't really know who in my school's families had money and whose didn't until we started driving and a few classmates had brand new cars and the rest of us had old used cars or borrowed our parents' car.
I didn't realize how much clothing kids who didn't wear a uniform owned until I got to university with two suitcases and all the other girls showed up with trunk loads of clothes! My mother says the uniform was definitely a cost save for her (even if my school fees were not!)
I would seriously hesitate to send my child to a school that did not have a uniform.
As far as gender-specificity for a uniform goes, I agree with the many posters that say all kids at a school, at least once they are 11+years old, should be allowed to choose between all uniform options, which should include a knee-length skirt option, a trousers option, and perhaps a knee-length shorts option and kilt. Knee-length was always easy to achieve because my mom would buy the skirts extra long and put in the first hem to the knee as a very deep hem (no trimming off the extra length), then just take out the hem and let it down a bit every time I grew taller. These skirts were made of this indestructible royal blue canvas and lasted forever. They actually became better with age because they'd soften a bit which was more comfortable. Everyone hated having to retire an old skirt for a new one and having to "break it in" again. We even experimented with one in chemistry class. It wouldn't burn or melt or scorch, and even hydrochloride acid barely made a mark on it!
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Short version:^ As a student, I loved wearing a uniform for many reasons and strongly advocate for the reasonable use (and subsidy, as needed) of them in schools.