Ask for a meeting with the head, not the teacher.This is important: be seen to be taking notes in the meeting. In the meeting ask for a copy of the school's anti-bullying policy. If it is not available for you ask when it will be given to you.
Show the HT the clothing and tell your child's account - explain that this seems to be an assault that has taken place, and ask what supervisoy staff noticed or did about it. Explain that the nature of the assault is serious enough for the police to be involved, and if you do not feel assured by the HT's reactions then take your notes and your clothing to the police.
This is assault. Bullying too, but assault.