My daughter did the same thing in second grade. Bullies picking on her...she decked the kid in class. I had to meet with everyone. Long story short my daughter was allowed to stay in school and the teacher was told to watch the boys in the class more closely. My daughter was a competitive swimmer and by 4th grade was swimming underwater for the 25 yards faster than these boys could dog paddle. She was also stronger and faster than most of the boys so they did indeed quit picking on her.
Long story short, she enlisted in the Air Force and you know that obstacle course they have to run at the end of basic, well she won, beat all the guys in her class. She also played on men's soccer teams in CA and Germany. If she was ever bullied again she never complained about it. She graduated with honors from Akron U and I couldn't be prouder of her.
Sometimes being bullied just makes you stronger. Good luck to your daughter.