When /etc/sudoers is modified to authenticate with user's password, kdesu also authenticates only with user's password, but the pop-up window text mentions incorrectly the root password.

In order to give every student in the lab control over their workstation without needing the root password, I modified /etc/sudoers with:
  User_Alias FULLADMIN = clange,slange
  Defaults:FULLADMIN authenticate
  FULLADMIN  PLACE = PASSWD: ALL

Since kdesu wraps around sudo, it also requires the user password to authenticate. This is a feature of kdesu, not a bug, as explained in Bug 339922.

But sudo informs correctly which password is being requested:
  > sudo mount
  clange's password:
(or slange's password  if slange is logged in) while the kdesu pop-up window misleadingly asks to "Please enter the Administrator (root) password to continue."

I suggest that the pop-up window mentions the proper user, whose password is being requested, as with sudo in the command line.