This may be the case in some areas, but in the most violent schools, they most certainly do not make them safer.
A study in Texas found that the introduction of school uniforms in the most violent schools caused cases of violence to increase. The same occurred in schools in Miami. What works in some areas, does not work across the board.
Also, while they may reduce the number of violent incidents in schools, they only make them worse outside schools. School uniforms make students a target for aggressors from other schools and can exacerbate inter-school conflict and violence.
Uniforms only make intruders easier to identify if teachers are subject to a strict dress code as well. Without a teacher uniform, intruders will simply be mistaken for faculty and will gain access to the school and classrooms.