Are the user logins connected to the email address I entered for them when I uploaded the class roster?
If so, when someone changes their email, do they have to create a new login/password?

No. You putting in an email address for somebody doesn't really do anything other than allow you to "blast email" all of the email addresses you have in the system to invite people to your new site. When your Classmates actually sign up for your site, at that time they will enter their own email address (even if it's the same as what you provided) and create their own private account.