aaabbbcccd
For starters, "command not found" does not refer to your variables, which are directories and a numeric value, not commands. And the "exit 0" is not going to cause 3 "command not found" errors. Most likely, some commands are not in the path of the user that is running the script.
Also, I noticed this on line 28:
echo "Please enter a student username to grade: " Are you copying bits and pieces from different homework assignments?
Personally, I would've used "case" instead of "if"