CD-R's are just that, recordable. Once you write stuff to them? it's permanent, especially if you finalize the cd.
Now with nero, they have this cool thing with those "multi-session" cds. You can actually write more stuff to a cd time-after-time until it fills up.
To do this, when you burn the cd, make sure it's set to "start multi-session cd" or if you're continuing one "continue a multi-session cd" and there ya go. make sure you also save the state of the cd inna .nrg file or something like that. Now note, you can't have the files in the same place with the same filename, so maybe create folders by dates you burn the files to the cd. It'll keep the cd organized, and then you'll have multiple-dates for the cds.
Since it's just basic schoolwork, I'm sure you won't be using big files. so this multi-session cd thig should work great for you man, try it.