Hi,
is it possible to automatically use the E-Mail address as username?
I tried
but the pupils can select anything as username
I am actually running into the same issue.
People even can use Usernames with blanks inside:

And this is what every admin don’t want users to do at all 
Maybe there is a need for two feature requests:
- set default user name = email address
- check user name if there are forbidden characters
Community, what do you think?
There is already a github issue: User name pattern instructions · Issue #142 · nextcloud/registration · GitHub dealing with similar question