Is it possible to replace the Mac's Login Window?

Solution 1:

I don't see any option for customizing the Login Screen other than the ones you have already mentioned. Based on Apple's protective nature, I wouldn't expect there to be much in by way of writing a custom login screen, but if there were I am sure it would have to manually be written in Objective-C Cocoa.

For Senario 2 I would say you will also need to create a custom script or app, which prompts the student for some credential, then mounts their home folder. Try Applescript.

Do 1st through 3rd graders really need their own logins? Their personal passwords are going to be terrible. Maybe it would be better to teach them to just put their files in the folder with their name on it.