Hi Debbie,
You can do this with aliases/nicknames, but it is still dependent upon the student to manage unfortunately.
However, this does not automatically send all mail as the nickname. Instead, it creates a drop-down option on the "from" field of a message and allows the student to decide on a message by message basis which account they would like to use to send the message. See the screenshot here:
You are probably best off to change the primary email address for your students to avoid this issue. That or just turn off the ability for them to send mail outside of the domain. Most of the districts we work with follow nomenclature along the lines of first.last##@
school.edu for student email accounts.
Good luck! -Joe