Talk to your child about why he feels he is being disrespected in class. Ask him who his friends are at school and how they treat him. If your child has no friends, talk to his teacher and see if there is a nice child in his class who might be willing to be his buddy. Talk to that child's parent and organize a play date.
Develop a solution to the problem in conjunction with your child. Ask him what role he wants you to play and what kind of a solution he would like to see. Your child may ask you to step in and talk to the teacher, or to stay out of it altogether. Let your child cry and offer him the emotional support he needs. Work together to come up with a list of tactics and options for dealing with his classroom situation.
Meet with your child's teacher if she wants you to do so. Explain the situation in a constructive, positive manner. Avoid berating the teacher or blaming him. If your child has identified problem students in the class, ask the teacher to deal with those students specifically. Work with your child's teacher to develop solutions. Ask the teacher to keep a close eye on how your child is being treated and to intervene if any sort of bullying or teasing arises.