How about "Hi (name), I hadn't heard from you regarding the student ID photocopy which I sent to you, and I wanted to make sure that you had received it. Have you?"
This gives them the opportunity to explain the delay, but does not sound accusing or complaining.
It might just be that they are slow in responding, or it might be that there is a problem. Either way, you can't make them answer your question, you can only remind them that you are waiting and give them the opportunity to respond.
You didn't mention whether the request was made in person (by a specific person who you have access to) or via email. Either way, I have found that the best way to get my questions answered is to be polite (as I can see that you are already aware) and to make it easy for them to answer (by contacting them).
Another incentive is to make them feel like they are doing you a favor by answering. You could add "I don't mean to be a bother, but I have been feeling anxious about the situation, and I was wondering if there was something more I needed to do in order to comply with store regulations". This asks for help, and shows the person in charge that you are willing to comply with their rules. Both helpful things.
It may be that whatever is delaying the confirmation has to do with your official status. They might be afraid you are an illegal immigrant. It might be irregularities with your paperwork, or some official requirement that is yet to be filled. If any of these are the case, the delay may just be the result of the fact that whenever the government is involved, there will be delays. It's frustrating, but there isn't anything you can do and it might be that there's nothing they can do, either.