Another student accused my son of sexual harassment!

Georgia , My son is the leader of his high school band drum section. He recently told a girl who was talking repeatedly to run a lap, which is within his scope as leader. She refused and he reported it to band director (adult), again, according to school protocol. Director sent her home, and on her way she poured ketchup all over my son's car. He wasn't going to report it, but other kids, who witnessed it, did. She was suspended from school for two days and from sports for twenty days, which kicked her off of the volleyball team. She is know to be dishonest. Her parents have consulted with a lawyer, and wrote me a letter stating that she was going to start a diary of his "sexual harassment" of her, that she had been harassing him for the past 3 years, that he had pushed his desk into hers, called her a drumline whore--and that this was what kids called her since most drumline kids are boys. They want action taken against my son or threated to bring suit. How should I respond? I thought a letter, describing recent events and denying her allegations. Should I do more? Don't want to spent $ on this. A lawyer who suggested mediation with her present wants $250 for it. She says they could arrest my child, bring him to juvenile. What could they do? Any other advice would be great!