Hi mommies, I feel for you cause Im a mom myself. Yet, I work as a counselor so I just hope to share some insights.
NO matter who may bully your child, ask your child for the details, without commenting or jumping to conclusion. We parents need to see and understand it from our children's perspective.
Though we know ur child is really troubled, talk to the school authorities, BUT not the other child's parents. Talk to the teacher, counselor, the coordinator, the school directress/principal about the incident. They are the best persons to handle the situation. Our concern is the welfare of our child. Whatever justice we want, best to trust the school about it.
Lets choose what is in our control and let go of what is not. What can we do is that develop our child's self-esteem, confidence, interpersonal skills, the kind of friends to play with and the ability to deal with others. THis lies in our hands as parents. The values we teach at home are what manifest in school. If we forever save them and cushion them from difficult people, the opportunity for them to learn to deal with them apprpriately us missed. The more the pattern continues, they dont get to develop their personal skills.
Ive seen students with moms fighting their battles for them. They are less resilient and cannot rely on themselves when they later move to middle school. They are become weak or bossy too, knowing that others would be there to fight for them. If they go to my room, telling me of their concerns with classmates, I dont mediate for them. I coach my students on the ways they can handle it. Ive seen cases too, that parents fight due to their kids' misunderstandings. Such a sad reality.
So to avoid all these, trust your school. Follow up if needed. Ask for an appointment with school authorities. If your child is uncomfortable, explain to them that you want to ask help. Best to do it in a tone of concern for the welfare of your child, and how the bullying/teasing is affecting him. This is not about the difficult kids. Its the "behavior" and not the person of the bully.
Yet in the end, we are teaching our kids Life Skills. They bring this with them for life. Everywhere there are bullies, difficult people and all. But we can help our children manage them if we develop the skills needed.

For more help, you may read:
http://www.smartparenting.com.ph/preschooler/behavior/help-your-child-deal-with-classmates-who-teasehttp://www.smartparenting.com.ph/preschooler/behavior/how-to-deal-with-a-bossy-classmate