Optiplex 210L Login logoff password field issues, running xp

I run a computer lab at a high school and we have a problem with our optiplex boxes of the 210L model. We have a situation that when you log off of a Student profile, that has limited rights, to the Teacher profile, that requires a password, we are not allowed to type into the login field. this happens sporadically, but is really frustrating. It seems to be only on these models in our schools. Is this a Dell thing? I mean, we tried the XP route, to see if it's an xp issue, bu then wouldn't it be on all our models it happened?