Second of all, parents will save money if students wear uniforms. The parents won't become poor because they can buy a few sets of uniforms for their children and not a lot of other clothes. Kids could even just have one uniform, so the parents won't have to pay too much. Then parents can spend their money on things the family really needs instead of a lot of clothes for their children.
Lastly, uniforms will help everyone be able to tell who is in a class when they are on field trips. For example, if a kid is lost on a field trip, you would be able to spot the kid easily from the other students because of his uniform. The whole school would have the same types of uniform, so the kids would all match the other kids from the school. It would be easier for students to find their class on field trips, too. They would just look for the school color.
I really don't think individual clothes are as appropriate as uniforms for school. Why, you ask? Schools with uniforms look more serious, it saves families money, and it helps to make field trips more peaceful and safe. Bye bye, personal clothes. Hello uniforms! How could you not be convinced?