Do students wear school uniforms for safety reasons?

That is at least one of the stated reasons. However, most schools which use uniforms do so for other reasons, whether it is religious reasons or because it seems to make teachers' jobs easier.

In some cases, uniforms might help with safety. Wealthier students might be less likely to get killed over their clothes. If students are members of rival gangs, they won't be able to wear their colors and distinguish themselves. But it won't help in the case of gang signs. You can just as easily make gang symbols with your hands in a school uniform as you would in a sagging pair of pants made by some urban brand. It is ironic that some of the urban brands also make work and school uniforms.