A school uniform is a standard set of clothing students wear when they go to some schools. It might have a particular color of trousers or a skirt, plus a matching shirt and perhaps a jacket or necktie, with matching shoes.
In some countries, students can wear anything they like when they go to school. In other countries, there is usually a standard dress code in school, usually a set of dressing for girls and one for boys. Boys and girls need to wear school uniforms when they go to school.
In many countries, some schools require wearing a uniform, and some do not. but still, after all this school uniform is the most important part ever.