From the articles I've read regarding issues with dress codes, problems usually are brought up because of articles of clothing that are on the border, NOT because of articles of clothing that you mentioned that are pretty obviously inappropriate.
Thus conflicting opinions of where the LINE for what is appropriate emerges.
For example, this girl's sweater dress
http://www.nydailynews.com/news/national/ky-high-school-teen-kneel-dress-code-violation-article-1.2505451
And this girl's top:
http://www.refinery29.com/2015/08/92540/dress-code-violation-exposed-collarbone
This girl's khaki skirt:
https://www.yahoo.com/parenting/high-school-teacher-publicly-shames-student-for-160356356.html
And this girl's prom dress:
http://metro.co.uk/2015/01/30/american-school-girl-made-to-cover-her-shoulders-at-high-school-dance-5043143/
This girl's leggings:
http://www.today.com/parents/sister-slams-high-school-viral-post-after-girl-sent-home-t13721
I don't know about you, but all of these look modest to me. I don't see how anybody can be distracted with any of these outfits. Like do they need to wear a hijab so that other kids can control themselves?
Not everybody plans to work for the government, either. The argument doesn't really make sense to kids who don't plan to work in a business office. It also doesn't really make sense to say, "Because I as an adult am limited, younger kids should be, too." Why shouldn't kids have the joy of wearing what they want while they're still young before it is taken away in adulthood? I agree that to a certain extent people should prepare for the real world, but I'd also want kids to enjoy being kids without getting all suited up just to go to school.
Don't get me wrong, I completely agree with the idea of there being limits, and agree with the fact that a lot of things (tube tops, thong showing, bare midriffs, etc.) are not for school. But it seems that a lot of schools are going a bit coo-koo with the idea that a girl's body is so sexualized that other people can't handle to even look at a shoulder, collarbone, and knees without it being a distraction and call for attention.
People are generally not against dress codes. They are against people who enforce it so strictly that girls who are wearing normal clothing are being sent home and sometimes even humiliated for it. That's the issue here.