In this day in age, the easiest way to communicate with your child’s teacher is often through an email. Not only it is usually the quickest way to get in touch, but it’s also usually the quickest way to get a response.
DO make sure you sign the email and sign it how you want to be addressed. Sometimes I will get an email from firstname.lastname@example.org without a parent signature at the end. Then that leaves me having to respond, asking who it is that is sending the email.
Do start off the email with something positive. When your child comes home with something that you are upset about, this can be very hard. As a parent, I have written an email, saved it, and edited it the next day when I’ve cooled down.
Don't automatically email the principal. If you are having a problem with the teacher, try emailing the teacher first and see what the response is. If you are not satisfied with the response, then you can work your way up the chain of command.