your main users table needs to have the user level column. for students, this column will be set to 1 for non students, set that column to 0.
then in the authenticate user server behavior on the login page, go to the third step and set the user level column to be stored in a session on login.
from there, you are on the right track of creating the rule to allow access if the userlevel session is equal to 1.
for details on this, see the userlevel tutorial on the security assist support page.