AITA for not wanting to drop charges on a 10 year old kid that beat up my 9 year old at school?

Some background my kids go to a school where over 90% of the student population are refugees/immigrants. We live in a neighborhood that is the same circumstances as my kids school.

Cut to yesterday. My 9 year old girl was out at recess. She was playing basketball with friends. This bully walked up behind her, pushed her to the ground and when she was getting up the kid kicked her in the head like it was a soccer ball. My wife went down to check my girl out of school. She'll be ok, I hope, but she has scratches on the side of her face, a black eye and it's swollen.

School principal said he'd handle it. Later on he called my wife and I separately and asked us to drop the charges because the student doesn't "understand" that what they did was wrong. I told the principal that if the school isn't going to discipline the bully I will call the police department and files charges.
AITA for not wanting to drop charges on a 10 year old kid?