A 26-year-old former worker at a children’s ski school at a Lake Tahoe resort, has pleaded not guilty to charges of child pornography and sexual exploitation of a minor. Stephan DeGraffenreid of Gardnerville entered not guilty pleas to four federal counts during a hearing Thursday afternoon in Federal Court in Reno. A hearing has been set for April 18 to determine whether he should continue to be held without bail until a trial set for June 13.