This is a list of "Do"s (e.g. use a salutation and formal closing) and "Don't"s (e.g. include emoticons or texting abbreviations) for how students should email teachers or other adults outside of their family or circle of personal friends. It includes a good and a bad example, plus a list of times when it would and would not be appropriate to email a teacher. (Feel free to edit that based on your own preferences for communication with your students.)
I recommend handing this out and going over it with students at the beginning of the school year, and perhaps also posting the document online if you have a class website with other resources they can refer to. You may wish to have students email you (perhaps about a fake topic) as a homework assignment to practice doing it correctly. With this in mind, I have also included a simple grading rubric with which you can assess how well they have followed the instructions.