To write a note

[MyNameHasBeenTaken]

To my dd headmaster?
She came home today saying 2 boys had called her a bitch.
They did the same to her (male) friend last week. Along with pulling his trousers down in the playground.
These kids are 6/7. Infants. Key stage 1.
I was upset for the little lad last week with the trousers issue. Now upset for dd with the name calling too.

Maybe a word with class teacher first?
Obviously the school will be discreet about other children than your own, but you may be adding information to other facts which they are gathering.
If they are inclined to brush it under the carpet, and come out with fatuous comments like "boys will be boys" (I sincerely hope not, but can still happen), the words "child protection" and "safeguarding" often have a galvanising effect.

Did she tell the teacher?

Is there a reason you want to bypass her teacher?

Teacher first. Only escalate if you don't get an adequate result. It's unacceptable behaviour but go via the proper channels.

Sorry
Drip feed alert!
Her teacher is off sick at the moment so they have had a collection of stand in teachers