In many countries, pupils are required to wear uniforms when attending school, and this is something that should be enforced for the following reasons.
Primarily, students, especially those in their teenagers years, are under pressure both academically and socially; insisting on a standardised uniform within the school alleviates some of this as it means that the student does not have to worry about whether their peers think that they are dressed well or in the latest fashions. This has the added advantage for those from lower income families who may simply not be able to afford the clothes worn by those from more affluent families.
Not only does it remove the social pressure, but wearing a school uniform also has a psychological advantage, as the student is dressed in the mode of attire used for study. Much like wearing a suit and tie may signal going to work for many people, wearing a school uniform prepares the child for the day of learning.
However, this perhaps works best for younger students. By age 16 or 17, it may be appropriate for the student to wear more casual clothing when they reach their later teens, as the idea of 'conforming' by wearing a uniform could discourage the student from pursuing further education and this is often the time in which young adults are looking to establish their own clear identity. It is at this point that it may be more productive to allow more relaxed clothing such as jeans.
In conclusion, although a school uniform does have advantages at certain stages of a student's academic career, there is also a time when the enforcement of a standardised set of clothing may be counterproductive.