Thank you for your email. I understand that you would like her to be more assertive around other children.
This is a good time to have the appropriate touching talk with her.
Ask her questions and help empower her with words she can use to protect herself. Start by talking to her about how her and her body are precious and discuss ways in which she can care for herself. Discuss appropriate touching, such as it is ok to give mommy and daddy hugs, but some children at school might not liked being hugged. Why?
Is there anything the children at school do to her that she doesn't like?
She may respond by saying pulling and pushing.
How can you tell the children at school that you don't like it when they do that?
You can say NO PLEASE DON'T PUSH ME! In a strong firm voice and put your hand up, or STOP PULLING ME IT MAKES ME FEEL SAD! Move back away from them so they can't touch you, and ask the teacher to help you.
Physical retaliation is never an option when dealing with aggressive behaviour as it exacerbates the problem and perpetuates the cycle of aggression. Speak to her teachers as well so they can keep an eye on her and intervene if they see it happening. Also include in your talk about appropriate touching that no one not even mom or dad are allowed to touch her private parts, and that only she is allowed to touch her private parts.
Good luck! Let me know how it is going.