Parenting

My child is being punched at kindergarten by a girl who also continually does things to him. For example, putting soap on his chair, messing with his outdoor clothes, etc.

The teachers know and have talked to the girl's parents, but the behavior continues. My child is scared to go and cries about it sometimes. I'm not entirely sure how to handle it, because that's the teachers job and I'm not there to help or see it.

Eventually, I'm considering involving the police, but that seems pretty last resort.

Any suggestions?

Edit

The director and teachers are all aware of the issues