Without knowing the details of the mistreatment, if I had felt it was severe enough to justify a change of schools, I would also send an email or letter to the principal (in writing) to explain your understanding of the incident and request an investigation.

It's possible another staff member witnessed it or that the person responsible will admit the incident and provide their perspective.

DS10 (DYS, homeschooled)
DD8 (DYS, homeschooled)