Of course it would. It would help decline the ever growing rate of oversexualization also. As a current student, I can promise you that 95% of students don't care about how much skin others show - it won't affect your concentration at all. Sorry to break it to everyone, but students don't care if one of their peer's skin is showing. It's a normal part of life, but the school system is teaching us to be ashamed of our bodies. By not having a dress code, students would feel more comfortable because they are wearing what they're confident in, rather than what's left by the school rules that are becoming more and more excessive. Also, many schools have horrible heating/air conditioning, so in the spring or early fall, it's physically painful to wear long pants. You're increasing the chances of dehydration (because let's be honest: school water is always disgusting) and other health problems. The people who claim that dress codes help kids concentrate are wrong, and they are helping create an even more oversexualized world.
No, abolishing a dress code would not help kids concentrate. If anything, it would do the exact opposite. Dress codes prevent kids from wearing distracting clothing. To abolish a dress code would bring this distraction right back, so to ask if abolishing a dress code would do away with this problem is asinine.
Lacking a dress code would not help kids concentrate because of the fact that children would be much more prone to losing focus on tasks at hand due to distractions involving clothing options. A dress code ensures that certain things that are distracting are not around to be a distraction, and would promote concentration.