What grade? Well, this involves chemistry, but it's simple chemistry. Access to a chemistry lab would be very useful.
The normal analysis for ascorbic acid (Vitamin C) involves titration with iodine. So you need access to iodine, but the quantities involved are small. Iodine requires careful handling and good practice, but I wouldn't think that is too much for a high school student. The titration is done with a starch indicator, and the end point is a very dramatic color change. I've done this titration many times, and I think it's fun.
It would help to start with some pure vitamin C, readily available, and be sure you can get the right answer with your technique.
It wouldn't surprise me if orange juice was variable in its initial Vitamin C content. Something you might check and report as part of the study.
As for the study itself, it is an interesting question having a rich folklore, so there should be some interest from your audience.
The destruction of Vitamin C is probably oxidation and the source is oxygen in the air. So, handling is critical. You will need to carefully record how the juice is handled, how often it's poured, how it's stirred and how often, how it's stored and at what temperature. Any handling is likely to promote aeration. The container is important. Full? Covered? Material? Oxygen diffuses through polyethylene but not through glass.
Since all these factors will influence the outcome, this could be a great way to learn technique and demonstrate that you have it!