That's not self-esteem. Self-esteem is a person's sense of worth. It comes not so much from what is worn but how they feel about themselves. A child who is told all day that he is stupid or clumsy or ugly will not feel better because he is dressed like everyone else in the classroom. Uniforms will not fill an empty stomach.
Hunger is a bigger obstacle to good schoolwork than clothing.
Uniforms will not correct a chaotic home life. A child who comes to school sleepy because of trouble at home will make little progress, even if she's wearing her pleated skirt. Uniforms will not correct learning disabilities. Nor will they make uninterested teachers become more interested in helping children learn. Forcing high school students to wear uniforms will make them find other ways to state their individuality.