Per my experience reports will not work to the advantage of the victim and neither will public discussion. I had been a victim of sexual assault back in high school and never reported the case. In the early 2000s it unfortunately provided too much controversy for the victim to handle and often times the case would lose. There will also be other students and teachers who get involved and that will impact how the child is treated in school.
If enough time has passed without evidence, there is nothing to back up the claim.
The best thing to do for a single case scenario of this kind would be for the victim to find a single case of peaceful revenge to ease their conscience and move on. She could reach over the harasser when he tosses lunch in the trash bin or something to prove that she is strong and unafraid. If he grabs her, she could slap him in the face or grab his wrist and call him a creep and tell him not to touch her. Perhaps then the bully will stop.
Another thing to do would be to go to a school counselor and see if they have resources or advice for sexual harassment without having them intervene between the students. Perhaps they will find material to make her feel empowered.