"school uniforms reduce/increase incidences of bullying"
Some suggest that school uniforms reduced peer pressure, by imposing a sense of equality upon students, but it's questionable whether this resulted in less bullying. Anyway, this argument is discussed in more detail under equality.
While school uniforms may reduce some kind of peer pressure, they increase other types of peer pressure between students. There is little or no evidence in research that, overall, school uniforms did reduce bullying.
The point remains that any mandatory dress policy is restrictive, i.e. it rules out clothing that may in some way reduce bullying.
It is this point that makes the bullying issue into an argument against school uniforms.
"School uniforms reduce peer pressure, in particular discrimination against students who wear cheaper clothes, or who wear the same clothes the next day." - anonymous
"Few people will notice that a student comes to school wearing the same jeans and T-shirt the next day. Anyway, jeans and T-shirts are so cheap when bought off the shelf, that one can easily buy a few of them in different colors and styles, for the same price as a uniform." - anonymous
Uniforms only accentuate differences in length, hair color and other physical characteristics. Children consequently judge each other by their physical appearances. One can argue whether it were better if children judged each other by their clothes instead. [source]
Uniforms make students focus more on faces, body shapes, sizes and color. The question is what kind of discrimination is worse. In regard to wealth, one can ignore discrimination, because wealth depends on one's parents. By contrast, body shape, size and color is much more personal. If one were allowed to choose one's own clothes, one could wear clothes (even cheap clothes) that draw the attention away from features that one fears might lead to discrimination. Uniforms deny students the possibility to express what they want to be in what they wear. [source]
When the ability to draw attention on clothing is taken away, students will focus more on expensive gadgets, such as iPods, mobile phones, watches, calculators and sunglasses, on sport clothes and equipment, on expensive food, candy and drinks brought to school or bought at the school cantine (Gatorade), and on how expensive the car is of the parents who pick them up or drop them off. It's much easier to look good in less expensive but well-chosen clothes, compared to the rivalry with gadgets, expensive food, cars, etc. [source]
School uniforms give students just another reason to bully each other, e.g. with comments such as: "pull up your socks" or "tuck in your shirt" or "tidy up your tie". Your assumption that uniforms lead to less peer pressure is not supported by studies into such questions. [source]
The uniform makes the student an easily identifiable and predictable target walking down the same road every day at the same time. [source]