In that case changing URLs won't help you at all i'm afraid.
As fare as I know your only option would be to include headers in all images, HTML documents and everything else you can download as teacher.
html:
Code:
<meta http-equiv="cache-control" content="no-cache">
<meta http-equiv="pragma" content="no-cache">
or getter yet PHP:
Code:
header('Cache-Control: no-cache, must-revalidate');
header('Expires: Sat, 26 Jul 1997 05:00:00 GMT'); // Date in the past
header('Pragma: no-cache');
There's of course autocompletion issue. you probably want to disable that as well
