I completely understand, my son is roughl the same age/grade and I too am beside myself trying to resolve this problem. My son cries when he has to go to school, not just at school. I feel your pain and wish we both find answers and our sanity
I am sure it is a combo of many things, you getting remarried, being away from you, being teased at school never helps. I would talk to the teacher about it and see if she can get the students to tease less, but you know how kids are!