@dupontr1 have a solution for you. You will have to use the preregister plugin. This plugin is designed to prevent spam registrations but it will work for your needs as well.
At line 259 of action.php, in lib/plugins.preregister/, at the very top of the function send_link(), insert the following code:
if(!preg_match("/@utt.fr$/", $email)) {
msg($this->getLang('bad_email') . $email);
return false;
}
This will check to see if the email address ends with "@utt.fr" and reject it if not. You can insert whatever message you like in msg():
msg("This site is for students of xyz university only", $email).
French is fully supported in the language files that accompany the plugin.