I can so relate this. My ds is 4 and was lashing out both physically and verbally a lot towards the end if last year.

I finally got it out if him that a daycare child was hurting him, but as no-one ever saw it, nothing was done. My dh worked out who the child was afternoon when ds said goodbye and gave a hug to everyone else, and shunned this child (while I don't condone exclusion on an ongoing basis, it was obvious this child had done something this day, as ds normally does say goodbye and talk about this child)..

We changed daycare centres earlier this year, and he is a different child even after a few weeks. He asks to go, doesn't cry at drop off, etc.