Federal agents, tipped off by Canadian border agents, have arrested a teacher at the Long Island School for the Gifted in Huntington Station on child pornography charges, officials said today. Michael Kelly Reiner, 46, of East Meadow, was charged with possession of child pornography, but federal officials said they were investigating whether he had sexually molested boys or acted out on sadistic fantasies. Federal prosecutors said their investigation into Reiner began in August when he was stopped by Canadian officials crossing from the United States into Canada in Ontario. "Reiner stated, in sum and substance, that he had a...