To add text to the mac login window use the LoginwindowText string in /Library/Preferences/com.apple.loginwindow.plist as in:

# defaults write /Library/Preferences/com.apple.loginwindow LoginwindowText -string “Classroom Equipment Support – Phone Ext: 2112 or 413-597-2112 First time login to this computer may take a few minutes to build a new profile. Questions-Suggestions: [email protected]”