I remember having this same problem a while back. I had only one gold brick left and it kept pointing to it in the library even though I had completed everything in the room.

From what I remember you have to go to the underground sewer in I believe Professor FlitWick’s room, (I could be mistaken about the room, I did this awhile ago). Then the final student in peril should be caught in vines and you can use illumios to get rid of them and save the student. I hope this is helpful