How to block OS X to run only one application
The easiest way to get a mode on a Mac for students taking tests is to create a second user (my school has a second user called Take Test) where they only allow access to certain apps (TextEdit, Pages, and a Dropbox thing they set up) using parental controls. Then, just have students sign out of their main account and sign into that test taking user before they take a test.

To create that second user, go to System Preferences -> Users &amp; Groups -> + and create a user. Then, for that new test-taking user, hit the Open Parental Controls in the right pane, hit Limit Applications on This Mac, and choose the applications from the list which you'd like to allow the students to use while they take a test.