I noticed that during the 72 hours I have to get off campus, the class progress bar is empty. Is there any way for me to fill this or is it just something that doesn't matter?

You can do a final party
If you don't want to stay at the campus so long, you can just send your Sim back the actual neighborhood before the 72 hours end.

If you mean the bar that shows whether or not you are earning an A+ this semester, I'm not sure why we still have it other than it might be too complicated to remove it for the last 3 days. But I haven't found any way to fill it or any obvious point in doing so.

I don't think it means anything. They haven't got any more classes, so there is no need in trying to fill it. They have already got their diploma with their grades on it so you can't improve their grades or anything.
I think they just left it there because they were too lazy to remove it or, as sidneydoj said, too complicated..