Signing your students into TinkerCAD is very easy if your school uses Google accounts for Google Apps for Education or Microsoft accounts for Office 365.
Step One: Go to the TinkerCAD website and click "SIGN IN" (very important to make sure the kids click "SIGN IN" and not "SIGN UP")
Step Two: Have the students click "Sign in using Social Providers". This will bring up a list of Social providers that you can choose from. (Facebook, Google, Yahoo, and Microsoft)
Step Three: Now have the students choose the service that your school uses. NOTE: The very first time your students do this, they'll be presented with a dialog asking them to "Allow" the connection. Have them click "Allow". This will only happen once.