So apparently once of my student loans has defaulted and been sent to collections.
On the student loan, I checked off defer payments until after graduation.
I am still in school, at the same school, and have remained a full-time student.
It appears that for some reason the loan issuer defaulted the loan and sent it to collections.
I am in the process of getting in touch with the loan issuer to get it fixed, as it CLEARLY was their fault.
I am very worried about it affecting my credit report/score
What could/should I do?

That should be an easy fix. First off, has the info already reported to your file? It shouldn't have because I think you have to be notified about the debt first, before it being reported. If so, dispute it with the CRA(s) and get the info corrected with your lender. Once your lender fixes it, it should be all cleared up. Hope that helps!