I went to a school which had a 6th form attached, and they required you to wear a uniform, and actually the students tended to like it. It was a slightly different uniform from the rest of the school, same colours, but the braiding on the blazers was thinner, the jumpers were black rather than grey, the ties were single colour with the school crest on, rather than diagonal stripes like the rest of the year. It all added to an edge of extra class. Because the uniforms looked quite stylish, people liked them, when you were younger you looked up to sixth formers and thought sixth form would be quite a cool place to be.
The most popular/controversial was the skirts for girls: whereas the 11-16 girls had to wear pleated skirts that were supposed to be close to knee length, the sixth form girls' skirts were tight mid thigh 'office chic' skirts. Also whereas all make up was banned for 11-16 year olds, sixth form girls were allowed to wear a small amount of subtle make up. These skirts were very popular with the girls as well as the boys, there was a big rumpus when the Head of Sixth Form tried to get the skirts changed, her argument was that teachers were told to go round telling the younger girls to stop rolling their skirts up at the waist, but they were seeing sixth formers going round in near mini skirts. On the taller girls they were pretty much like clubbing skirts. But our fairly liberal minded Headmaster wouldn't change them.
I remember when we were considering our post-GCSE options and people were talking about whether to stay on for sixth form or go to the local college, the usual argument would be "nah man go to college man, you don't have to wear a uniform" but our uniform actually looked impressive and I liked wearing it. I particularly liked it when I was playing for the school cricket team, our Headmaster was a huge cricket fan and when he got the job he made sure we had deals with cricket manufacturers to do jumpers and black traditional cloth cricket caps in the school colours and he insisted that on Saturday matches even if we didn't come in our school uniform, we always brought our blazers. We would play some of the posh private boys schools who turned up looking like a mish mash and our guys would walk in to the pavilion in immaculate whites, all in the school colour jumpers with our black caps and school blazers on, we looked like a 1930s England cricket team.