A 7-year-old boy has been accused of sexual harassment after punching a fellow first-grader in the groin, but the boy's mother says he was acting in self-defense. Tasha Lynch said her son was being choked on a school bus, and has been afraid to go back to school in South Boston ever since. “I think my kid was right to fight back [after he was choked],’’ she said. “He wasn’t doing anything except protecting himself.’’ A spokesman for the Boston public schools confirmed the incident had been classified as possible sexual harassment, but declined to comment it.