You can delay sending emails in Gmail by scheduling your messages for a later time. I try to avoid sending work emails over the weekend. This is a great way to send emails at your convenience and have them arrive when it is more appropriate.
A good email to a teacher should have a clear purpose. It should be written in a polite and professional tone. Be sure to open with a proper greeting and sign off with your name. Here are some tips for writing a good email to a teacher.