This may sound foolish, but if you are writing an online exam, you might want to take a look at using Flash.

Another thing you could do is store the information in a cookie (encrypted?) and store similar data somewhere on the server for verification (a lot of bandwidth, I know). In a web application I recently worked on, I store the next page as a session variable. However, this application isn't an exam.

One thing you could do is maintain an array, or comma separated list, in session to determine which questions have been answered. If the user goes back and tries to change an answer, let them think that they've changed their answer, but check to see if this is a resubmit, if so, trash their response and move to the proper page.