There's a great new Chrome extension called Dmail, which allows you to set an expiration date on emails. I believe this was probably started so that you could email sensitive material, like bank details, with a little less fear, but I can see some great ways of using this in schools too!
When you add the extension you have the option to have the email destroy itself in one hour, one day or one week. The email will not disappear from the recipients inbox (how great would that be), but instead turns into a scrambled up jumble of letters!
Firstly you could send confidential information, which you don't want there to be an email trial for, like information about an incident or confidential information about a student.
You might also want to use it for 'your snooze you lose' type emails. For example, asking if anyone is currently free or sending out a cake alert email!
You may also want to send students emails that they don't necessarily need to have access to after the lesson.