A black teacher at Manhattan’s Beacon High School told cheering students to boycott the elite campus Monday after an incident involving a white Jewish girl sparked complaints about a racist environment. “What we’re asking of all students: Do not come to school that day to show solidarity for the support of not just students of color but all students at Beacon in providing a safe space,” said a teacher who identified himself as “Mr. Green” at Friday’s boisterous cafeteria gathering, which was videotaped. “Until the demands are met, students are not to return to school,” the teacher exhorted the crowd...