A Rhode Island teacher has been arrested on charges that he assaulted a student for eating a cookie in the classroom. Richard Koster, 49, of Cranston, turned himself in to police Thursday and was charged with misdemeanor assault after authorities obtained an arrest warrant. Police said Koster teaches at Slater Junior High School in Pawtucket and assaulted the 13-year-old boy on March 18. Authorities said the boy reported that Koster lifted him off the floor, pinned him against a wall, and yelled at him, after catching him eating a cookie. Police say Koster got upset after another student told him the boy made a face at him after he told the boy it is against the rules to eat in class.