It’s an entirely different kind of school rush. When Zohran Mamdani won New York’s mayoral election last November, Caven Wagstaff’s phone wouldn’t stop ringing for weeks.  Wagstaff runs his own firm, wrangling places for wealthy American families keen to land their kids a perch at tony British private schools.  Mamdani, as mayor-elect, turbocharged his biz. “It doubled,” he says now, of the 10 families per week who hit him up for his Anglo-friendly expertise. “They were saying to me, ‘We want