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A Boston elementary school is investigating a 7-year-old first-grader for sexual harassment after he struck another boy his age in the groin. But the mother of the accused said her son was fending off the other child, who had choked him in an altercation on a school bus on Nov. 22. “I think my kid was right to fight back,’’ said the mother, 30-year-old Tasha Lynch. “He wasn’t doing anything except protecting himself.’’

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