Since you don't want students to see other codes, that eliminates using Choice.
Haven't the time to experiment, but the fact that you can make a database activity visible only to the user who made the entry should be doable with a properly construct csv file for import. The tricky part is going to be, We do not know who the students are yet. You might be able to do multiple imports to a DB activity with an updated csv file import, but like I say, I just don't have time to play with it right now to know for sure.