How can I force the user to stay on page after starting the test?

I am writing a web application using PHP, Javascript and HTML. My application involves tests that the user has to pass over a certain period of time, and programming-wise, the application works fine. The problem is that at any point during the test, the user can decide to simply back out by pressing the browser back button or leave the page, and I want to prevent them from doing that. This is not unethical, because before the user gets access to these tests, there is a page that explains the rules of the test, so if the user doesn't want to, he doesn't have to start the test. Any ideas? Thanks!