I am not as versed in this area. but .... Maybe it is the Fast User Switching?
Turn off (if it is on) and then Login might go straight to username entry.
Or check first on your good configuration to see if it is On or Off....This is probably not the issue, but something good to check I guess...
Go to gpedit.msc
Go to Local Computer Policy > Administrative Templates > System> Logon
Find "Hide entry points for Fast User Switching" and set to Disabled
If you disable this and anyone leaves the lab without logging off, you will have to do a logoff command from cmd to get thier user out: shutdown -m \\<computer-name