Salesforce has still not corrected this glitch in their account-generator so students who click the form Submit button multiple times are still winding up with multiple accounts, all with the same username! This leads to tons of confusion and chaos.
So please stress to any of your students when they are getting their accounts, that they click Submit only once, no matter how long it may take to get a response.
And as always, it helps if you emphasize they read & follow all the instructions and the cover pages carefully. Then they will have zero problems.
If they DO run into trouble, they should check the Help FAQ first, then post to the Help Forum only if they can't find help in the FAQ.
As always, thank you all for your understanding and patience!