All charges against the former Sag Harbor Schools official Susan E. Guinchard Kinsella, accused of knowingly hosting an under-age drinking party, were dropped yesterday after the prosecution threw in the towel. Her attorney, Daniel G. Rogers, had argued from the beginning that the charges were deceptive in nature.“She was clearly not guilty,” he said after the case was dismissed in Southampton Town Justice Court. “We would not entertain any plea-bargain offers,” he said, instead demanding either acquittal or dismissal. He got the latter.“They could not find any evidence with which to prove their case,” Mr. Rodgers said. “We have been demanding a bench trial every two weeks. They kept saying, “We are not ready for trial. . . . This, in my estimation, was nothing more than political correctness on steroids. It is unfortunate because my client had to pay the price.”Southampton Town police arrested Ms. Guinchard Kinsella three weeks after a Jan. 27 party for her teenage daughter at their Barclay Drive residence on North Haven.