As a teacher, I can say that I receive a large number of emails from parents asking this or that. Though I respond to all there are certain emails that are far easier to deal with than others. My suggestions/guidance for how to write and format an email to a teacher:
1. In the subject put your child's name and, if you can, describe the reason you are emailing in a few words. If not, leave it for the body of the email.
2. In the body of the email try to be as short and sweet and possible. Explain the reason you are emailing: "I am concerned about my child's low grade." Say what you hope to get out of it: "I would like to schedule a meeting to discuss it with you. When would be a good time?" Say thank you.
3. If you have a complaint or an issue that you are upset about try to keep the email as professional and impartial as you can. Accusing, yelling at or otherwise degrading the teacher through email will rarely get you a positive or constructive response back. Instead, explain your issue calmly and tell the teacher what you hope to achieve by emailing them. Again, keep it professional.
4. Don't ask for information that is readily available already. Try to avoid emailing about specific grades on assignments. Almost all schools now have a way to access up to date grades online through a parent access grade portal of some sort. If you want to know your kid's grade on a test or assignment. Check there, don't ask the teacher. The same goes for upcoming assignments or tests. If the teacher maintains an active calendar or website check there first. If all else fails, ask your child about something. Emailing the teacher should be the last resort.
5. Make sure what you are asking for is appropriate. Don't ask a teacher to change your child's grade or to excuse them from a homework assignment because of x,y, or z. No teacher in good conscience can do those things simply because you are asking.
6. Be sure to indicate who your child is if you didn't in the subject area.
7. If you want a phone call back, put your phone number in the email and say when the best time to call is. Be realistic though, most teachers aren't going to call you back at 7:30 p.m because that is what works best for you.
To summarize: Keep your emails friendly and professional. Be sure to indicate why you are emailing and what you hope to accomplish. Make sure to say who your child is! If you want a phone call back, put your phone number in the email. Don't get angry when emailing and make sure what you are asking for is appropriate. If you follow these suggestions you will probably get a prompt and positive response from the teacher answering your question or dealing with your issue! If you are a teacher and have other suggestions for this list, let me know (firstname.lastname@example.org) and I'll update the post with them.