Hear out the Child
When you see that your child is in a bad mood after he/she comes back from school, inquire as to what is the problem. Try to be as polite, loving and caring as you can possibly be because only when a child feels loved, will he/she want to open up to you.
If there is a problem with the teacher, ask him or her what it is that the teacher does and how it affects your child? Try and describe to the child that teachers do not always have it easy. However, be careful not to ignite further anger in the child; he/she might think that you are siding with the teacher.
Call the Teacher
If the problem seems to be persistent without any real reason from the side of your child, it is a good idea to call the teacher personally. Be very polite in your conversation with him/her, while fully describing the point of view of your child. It is a huge possibility that the teacher might not actually be aware of the whole situation.
Teacher Parent Meeting
If there are any such meetings scheduled, good enough, otherwise one can be scheduled out of the blue as well. This is a good chance to have both the teacher and child sit together with you in the middle and see what the real issue is. You must play neutral and not take the side of either party in the matter. It is important that both sides must be comfortable in communicating with you and do not see you as being biased in nature.
It is important to take both of their statements into consideration before giving a suggestion of your own.