I've already discussed this before here but if you are looking to buy school uniforms but are worried about exploitation, you should think about all the school products we get that are imported from overseas, and how we can over-think this and realise that using our purchase choices is too indirect a technique to lessen crimes of exploitation. Exploitation in Australia tends to only be possible within the context of organised crime. In other words, to force someone to work for well under the going rate (with the job market the way it is) they would have to be locked up. It is ironic that the local prisoners labour on sewing machines.
I suggest when looking for school uniforms to search for the best quality. Quality demonstrates attention to detail and pride in one's work. This only happens where the sewer is comfortable, happy and satisfied with the conditions of employment. Unhappy workers can easily sabotage the interests of the employer without risk of getting found.