How to get rid of box outline on a placed image in Illustrator

I'm also a graphic design student who also had exactly this problem.  I had placed a linked image (not an embedded image) into my Ai doc, so the solution to expand the image didn't work because linked images cannot be converted into objects. 

I reached out to the design lab on my campus for help.  They were quite puzzled by what was causing the outlines to appear, but after exploring many possibilites, we found that by turning off and unlocking all layers, the outlines were still present.  When I clicked on the outline using the selection tool (black arrow), the word "Artboard" followed by a number appeared in the upper left corner of the outline.  I used my selection tool to click on the word Artboard, then pressed the delete key on my keyboard.  After pressing delete, the word Artboard appeared again, but with a different number.  There were actually FIVE artboards stacked on top of each other, each creating its own outline.  I have absolutely no idea how these artboards were created.  I had to press the delete key one time for each artboard.  After all the artboards were deleted, the mysterious outlines disappeared. 

Sounds like you just need to expand the resulting image after the image trace. Once expanded, use the direct selection tool (white arrow) to select that outline and delete it.

You can select the box with the Group Selection tool (white arrow with a + next to it). Be careful to click on the edge of the box, when making the selection. Then you can delete.

If the background of the image you are tracing is white, and there is no white that you want to keep within the image, you can expand the Advanced section of the Image Trace panel and choose Ignore White.