Remain calm at all times. Remember that your position as the teacher is essentially the last word. You will ultimately decide what is best for the child in your classroom, and you must not allow yourself to be bullied in your place of work.
Listen to the parent. Allow them the space to be heard. Carefully take in the information given in order to contemplate what should be focused on first. A parent can be upset and still give important information that will help you in educating her child. Don't disregard everything that she may have to say because of how it is delivered.
Assert your credentials and position in the classroom. Remind the parent of your training, and keep your degrees and certifications in plain view. Never allow the discussion to be held where you are sitting and the parent is allowed to stand over you.
Watch your body language and tone. While the parent may be difficult, you must remain in complete professional control. Do not argue points, interrupt or place blame on her. Never raise your voice at her and move to her level of behavior in the discussion. Keep firm eye contact but do not be afraid to turn away and move to another area of the classroom if you are being disrespected.
Tell the parent that you will be available to talk to him for a specific amount of time. This will force him into making their issues more specific and will also give you a very clear method of ending the conversation if needed.
Invite the principal or a counselor to attend one of your meetings with the parent. Having another person present may calm down negative behaviors by the parent. It will also give you someone to back up and verify your experiences.
Spend extra time with the student. Talk to the child in order to gain context as to why his parent is upset. Don't ask him directly about this. Rather, say, "When you get home, do you talk about school and get help with your homework?"
Provide nonconfrontational resources for parents to talk to you. Send home comment sheets and schedule open houses and parent-teacher appointments on an established routine. This will provide less opportunities for random parent appearances and will allow you to manage your classroom efforts better.