As a school governor, hopefully I can help a bit with this very sorry situation - I cannot think of anything worse for the OP.
Step 1 - This is a safeguarding incident so will need to go to the Schools Designated Safeguarding Lead (DSL) which should be listed on the schools website. Ask for a meeting as a matter of urgency, take support with you (a close relative for example) if you would feel it appropriate - notify the DSL in advance. Take notes and agree them/send them to the DSL - ideally keep them brief and to the point if at all possible.
Step 2 - If you are not happy with the DSLs response, book a meeting with the Head (or include in Step 1, but probably best to go through this as a separate stage so that there is a secondary look at the situation)
Step 3 - After that meeting, if you feel you are either not fully informed as to the incident and actions, then ask to make a formal complaint to the Governors - it most cases it would be best to talk to the nominated Safe Guarding Governor (there will be one) informally with a view that if you are not satisfied to lodge a formal complaint which will be normally led by the Chair of Governors.
This is a rough roadmap, each school should have published policies on the website (or if not, you can ask for them) - Safeguarding policy and Complaints Policy are the key ones.
Finally, and as a last step, if you believe the school are sweeping the issue under the carpet, approach the Local Authority Designated Officer (LADO) at the local county council offices who will be responsible for Safeguarding across the county and can work with you and the school to hopefully a successful and satisfactory conclusion that means this will never happen again and make sure that your child is appropriately supported.
Best of Luck…
A School Gov…