As a teacher you have the ability to request a teaching account via the Scratch website. After you have created your teacher account, you can then generate accounts for your students without the need for email addresses. You can set up the accounts with first name, seat number or any name that the pupils would like to use.
However, please do run this past your local information security officer to make sure that they are happy with this process.
https://scratch.mit.edu/educators/#teacher-accounts
