The best thing you can do is to keep talking to your child, communicating and counseling them, guiding them. Kids at school can be cruel, I remember when I was a kid and other classmates made fun of me for some reason, I was strong to put up with that, but there are other kids who are more sensitive and can be very affected by those situations. In my time it was not called bullying, but nowadays classrooms and teachers should be more attentive to these problems that are present in all schools.