Initially, go through the chain of command. Meet with the teacher. Be specific about Johnny's complaints. It is very common for kids to say, "My teacher hates me." This may be an interpretation of a one time moment, when the teacher had a bad day, right up to singling out this child and making his life very difficult.
Do not accuse the teacher, nor accuse your child of making it up. Remain neutral. Tell Johnny that all of you will meet and find out what is going on between the two of them. Approach from a "I know you would not bully Johnny, but he feels that the following examples of your behavior were hurtful to him. Cite examples; don't be vague. "You never call on him, " is not a strong case.
Do not put the teacher on the defensive. Be clear you want a good relationship between this teacher and your child; that is why you are meeting.
Allow the teacher to patch up the relationship with your child by sincerely defusing. explaining each accusation. If it is clear the teacher is not taking the relationship with your child seriously, do not say that to your child. Remain neutral-- say Johnny let's see what happens. If your child continues to complain, you meet with the principal.Explain what has transpired. Allow the principal to offer a solution: I will speak with Mr.Jone; Let's call Mr.Jones down: Are you sure your child is telling the truth...... You should be able to determine if this is a pattern with this teacher, based on the principal's reaction.
If the behavior continues, go to the superintendent of schools and demand the child be removed from this class.
Be your child's advocate but objective. If your child is lying, and you get the teacher fired, great damage will be done to your child's character.After years of working with children, many do not see situations clearly.