We have a few tips here to help with this common problem.

• Ask your child why? – Before you make any assumptions you must have all the information and you need to have an open chat with your child in regards to their teacher. Ask your child to explain what is happening? And is it a regular occurrence or maybe it was a once off? Let your child tell you why they feel uncomfortable and unhappy. You also need your child to be honest as if they are getting into trouble because they are talking or misbehaving you need to know before you move forward. 
• Write everything down and document dates- If it is a regular occurrence and your child is complaining regularly it would be a good idea to document the dates and what happened at school. Keeping record will help you to reach a solution with the teacher or the principal (should they become involved) and figure out what is the issue. Make sure you note how your child is behaving whether he is sad or angry after school, and how their feelings are being affected by school. Talk to other parents and find out are their kiddies having any troubles with the teacher or is it just your child struggling?
• Talk to your child’s teacher and be diplomatic- There is no need to barge in and accuse the teacher of picking on or being mean to your child as you have no real evidence and your child could be misconstruing the situation. Arrange a time that suits the teacher and you after school and rather than arguing try to be diplomatic and ask for their help in figuring out the problem. Accept that you may hear things about your child you would rather not hear and be open to the teacher’s side of the story. Be clear to the teacher that you are worried about your child and want her/him to shed some light on what is happening in school. In most cases there is a reasonable explanation and you have to accept that people have different personalities and maybe the teacher and your child aren’t bonding. By having this chat though you are letting the teacher you are keeping an eye on the situation and that is a clear signal that if the teacher is being unprofessional it should stop now.
• Consult the principal if not happy with the outcome- If after your chat with the teacher and a couple of weeks have passed and things aren’t improving consider talking to the principle to clear it up once and for all. Usually it doesn’t have to get to this stage but in rare cases it may be unavoidable if your child is feeling extremely unhappy in class. Again tell the principle the issue and how you child is telling you things when they get home and how he is deeply unhappy in class though he never had any issues before. The principle will have to investigate further and sort it out with the teacher, in some cases you might decide together that your kid will do better in another class with a different teacher and start a fresh. It may not seem the best idea but if your child and his teacher aren’t getting along it might be the only option, but try not to panic in most cases it never gets that far and things will settle down soon enough.