A school that requires uniforms usually works with one or two vendors to supply these uniforms. Consult with these retailers about your question. Sometimes special orders can be made. Consider finding a seamstress that can help modify the uniform to fit your girl's needs. Also, consider consulting the school's principal to determine if there is an alternative type of clothing for her to wear.
Sometimes writing to manufactures can help. Find out who makes the clothes and communicate directly with them. However, expect this to be a slow process.