CONCORD, N.H. — A graduate of an elite prep school who was convicted of having sexual contact with a 15-year-old classmate as part of a game of sexual conquest will be required to register as a sex offender for life, a punishment his lawyer likens to being branded and legal experts and reform advocates say exceeds the crime.Owen Labrie, 19, was convicted Friday following a two-week trial full of lurid details that exposed a practice at St. Paul’s School known as Senior Salute, in which graduating students try …