I had a similar situation with my eldest DD. Boy verbally abused her, (screaming in her face daily) in her first week at infant school. Being my eldest, I went in gently and got fobbed off for ages.
It's honestly my biggest regret I didn't go in more firmly and put it all in writing.
This boy got increasingly unpleasant and he escalated to physically hurting her, smacking, punching, wrestling her to the ground and ripping her coat off etc. I had several meetings with the teacher and Head, both were very defensive and told me it was nothing, my DD got upset too easily (!) After about 18 months DD was a wreck, always had tummy ache and didn't want to go to school. I dug around and found out the boy was literally terrorising the whole playground at break time. Turned out his Mum was a heroin addict. (She was always swearing loudly at the school gates.) His home life must have been awful, I felt bad for him, but he shattered my DD and many other children at such a crucial time in their development.
It's taken us years to build her confidence up again.
I would handle the same situation much more confidently if I had my time again.
So please keep pushing. Put your complaints in writing. Get the other parents whose kids have been affected involved. Do whatever you can to protect your DD from this abuse. The boy probably has had an awful life and maybe you can indirectly help him get on the relevant people's radar to get the help he needs.
Have faith in your DD and let them
know you believe them. Don't let the school play it down.
Don't be afraid to be a bit of an annoying parent.