I had a dress code in junior high similar to holly11short, where you had to wear a polo shirt (either navy blue, white, or khaki) and I think jeans. I think the arguments I heard at the time in favor of a dress code were mainly that it created a more professional atmosphere more conducive to learning (I'm pretty sure they didn't use the word professional, but I can't remember what terms they used now...), it prevented the development of a socially stratified environment, where it was obvious who had money and who didn't (people couldn't wear expensive designer clothes), and there were probably some other arguments I'm not remembering.
I remember at the time feeling against it on principle (I felt like people's personal expression was being suppressed), but honestly I'm not someone who cares all that much about what I wear then or now, so looking back it probably wasn't a big deal for me, and made it so I didn't have to think about what I was going to wear. For my high school, there wasn't a dress code like that (we could wear what we liked), besides the usual code about girls wearing shirts too low or skirts too high and so on.
Overall, I feel that my experience in junior high was of a much more constrictive, judgmental atmosphere than that at my high school. (My junior high was widely regarded as a 'good' junior high, whereas the high school I went to was considered 'west side' and ghetto.) As a result, I have a lot more negative associations with my time in junior high than in high school. At my junior high, teachers would stand outside classroom between classes trying to catch people who didn't make it in before the bell. I remember one time in particular, where I went to class (there were about five minutes between classes), and I really needed to use the restroom, but I knew that particular teacher had placed a ban on using the hall pass because so many students had been abusing it, disappearing for half an hour at a time. I spoke briefly with the teacher, and she looked sympathetic, but she couldn't make exceptions because that would cause problems (I don't remember if she actually said that, but I probably understood that without her saying it), so she just told me to run for it. For some reason, the restroom closest to the classroom was out of order (or maybe I just didn't know where it was at that time), and so I had to run to the other end of the school. I raced back, and I was literally about two yards from the door when the bell rang. One of the people standing guard yelled out threw out his yard stick to block me from getting in, because everyone who didn't make it class on time was always written some kind of citation. I was close to tears. In the end the teacher came out and rescued me, so I didn't get written up, but I remember being shaken and upset for a lot of the class after that, because I was a shy, sensitive student who never got into trouble.
So, I guess even though I understand why my junior high was that strict (students were horrible, honestly, and the hall pass was only one example of how they wantonly abused the rules), and maybe junior highs are just typically stricter than high school because that's a worse age for students getting into trouble, junior high holds memories of a feeling of oppression more than high school because of those stricter rules. In high school, I felt like I was in control, and I was a much better, more actively engaged student, with a much better self esteem, but in junior high, experiences such as that hall duty above who literally jumped in my way with a yard stick and yelled “Hey!” in a tone that made it clear I was as much a delinquent in his mind as the students who regularly ditch classes on purpose or wander about the halls, often had me feeling depressed and anxious.
So, given that experience, it wasn't really the dress code itself that created that atmosphere for me, rather it was a symptom of the school's stricter attitude. Strict is usually seen as better, but my experience was that strict also created situations that victimized the well behaved students who did want to follow the rules and do well.
*Edit: Although, again, perhaps that's just how junior highs typically are, because of the age range...(My sister had the opposite experience in the sense that she went to a junior high that was considered 'ghetto,' and didn't have a dress code, and her high school was considered a nicer, more well-funded school, but she had the same experience of feeling a bit like she was in prison in junior high, whereas in high school things were more lax.)
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Opal sets into motion her most diabolical scheme yet, to frame Artemis and turn his closest friends against him. Only this time she has a new calculating partner who knows Artemis better than he knows himself. [An Artemis Fowl fanfiction, set after The Atlantic Complex.] https://www.fanfiction.net/s/8336552/1/Noble-Heart
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