Because of the fact that you've already deleted the user before, without deleting his home directory, now there's no association between that user and that home directory, because the user simply doesn't exist. Therefore, the home directory has become a normal folder. If you don't associate it with another user, for instance by creating a new user and pointing to that home directory (useradd user
-d /home/student), then your only choice is to simply delete that directory with rm -rf /home/student.
On the other hand, if you haven't deleted any users and you think that the user associated with /home/student might still exist in your system, you can check /etc/passwd:
cat /etc/passwd | grep student
If no output is displayed, then it means that no user is associated with that home directory.