This is the gray area. Its a tough call to make, and sometimes you have to go with your gut feeling.
I would probably pull the egg and candle to try to re-access what's going on. I see a slightly darker spot on that upper left egg, but its hard to tell from the pic. If what I'm looking at is correct, it looks like it should be in the air cell. That should be fine, but its impossible to say for sure.
Since this is a class project for you, and you are learning, let me play Teacher for a minute and add some caution.
I'm not trying to scare you, just wanting you to think about the possibilities. Decide whether you are comfortable helping, and possibly hatching a duckling that is not ... um... "right". Will you be able to nurse it, cull it, dispose of it, etc or would it be easier to let Mother Nature make these decisions for you, and you just wait it out and see what happens. Also how you will feel if you help or don't help, and it doesn't survive. None of the options are right or wrong. Its a personal thing. I just want you to consider everything first.
It would be awesome if every chick, duckling, gosling, etc. hatched perfectly on their own, but sometimes they don't. And everyone has their own philosophies on what should or shouldn't be done. So its ultimately up to you.